Offering a 401(k) is one of the most common reasons small businesses start looking at PEOs. The pitch is simple: join a PEO, get access to big-company benefits without the administrative overhead. And for retirement plans specifically, that pitch is genuinely appealing when you’re running a 25-person team and have no interest in becoming a plan sponsor, fiduciary, or compliance expert.

But the details matter. A lot. The actual structure of a PEO-bundled 401(k) — who administers it, what it costs at every layer, how flexible it is, and what happens if you ever leave — can be very different from what you imagine when you sign up. Justworks is one of the more popular PEO options for small and mid-sized teams, and their retirement offering comes up constantly in the decision-making process.

This article breaks down exactly how the Justworks 401(k) works in practice: the plan structure, the fee layers, employer match flexibility, and the portability question most business owners don’t think to ask until it’s too late. If you’re still orienting on the basics of PEO services and benefits more broadly, start with our full Justworks review and PEO benefits comparison hub. This piece assumes you’re past that stage and want the retirement-specific details.

The Plan Structure Behind the Scenes

Justworks doesn’t run its own 401(k) plan in-house. The retirement benefit is administered through a third-party partnership, currently with Slavic401k. That distinction is worth understanding before you go any further, because it affects who your employees actually interact with, who holds fiduciary responsibility, and how much control you have over the plan itself.

In practical terms, your employees will enroll through the Justworks platform, but the actual account management, fund selection interface, and retirement-specific support runs through Slavic401k’s system. For most employees, this is fine — they set up contributions, pick funds, and largely forget about it. But when questions arise about account balances, investment options, or distribution events, they’re dealing with a separate vendor, not Justworks directly. This is one reason it’s worth understanding the Justworks PEO account management model before you commit.

The plan operates as a multiple employer plan, or MEP. Under a MEP structure, your company isn’t sponsoring its own standalone 401(k). Instead, you’re participating in a plan that Justworks (or its partner) sponsors, and your employees’ accounts are pooled with those from other Justworks client companies. This is how PEOs can offer retirement benefits to small businesses that couldn’t otherwise justify the cost of setting up their own plan.

The MEP structure has real implications. Fund selection is standardized across all participating employers — you can’t add a fund to the lineup because your CFO prefers it, and you can’t customize the investment menu for your specific employee demographics. The plan is designed for the collective, not for your company individually.

On the compliance side, the MEP structure means Justworks and its administrator handle most of the plan-level fiduciary responsibilities — things like investment menu oversight, nondiscrimination testing, and Form 5500 filing. That’s a genuine operational relief for small businesses. But it also means you’re ceding a meaningful degree of control over the plan design.

Roth 401(k) contributions are available through the Justworks plan, which is a meaningful feature. Employees can elect to make after-tax Roth contributions, traditional pre-tax contributions, or a combination. Standard IRS contribution limits apply: for 2025, the employee elective deferral limit was $23,500, with a $7,500 catch-up for employees aged 50 and older. The SECURE 2.0 Act also introduced enhanced catch-up limits for employees aged 60 to 63 starting in 2025 — verify current 2026 limits directly with the IRS or Justworks at time of enrollment, as these figures are updated annually.

Enrollment is handled within the Justworks platform, which keeps the experience relatively seamless from an HR administration standpoint. New employees can be directed to enroll during onboarding, and payroll deductions are integrated automatically. The broader Justworks HR technology platform ties these pieces together in a single interface.

Fees, Fund Options, and the Cost You Don’t See on the Invoice

Here’s where a lot of business owners get tripped up. The Justworks platform fee is visible. The 401(k) costs are not always as obvious, and they exist at multiple layers.

The first layer is the Justworks platform fee itself. Justworks operates on a per-employee, per-month pricing model with two tiers — Basic and Plus. Access to certain benefits, potentially including the full 401(k) feature set, may depend on which tier you’re on. Verify this directly with Justworks, because tier structures and feature availability change, and you don’t want to find out your employees can’t access retirement benefits because you’re on the wrong plan tier. If you’re trying to understand whether the overall platform justifies its price, our analysis of whether Justworks PEO is worth it covers the broader cost-value equation.

The second layer is the 401(k) plan administration fee. Slavic401k charges fees for administering the plan. These may be structured as a flat per-participant fee, a percentage of assets under management, or some combination. This is separate from what you pay Justworks for HR and payroll services.

The third layer — and the one most people miss entirely — is the fund-level expense ratio. Every investment fund your employees can choose from carries an annual expense ratio, expressed as a percentage of assets. These fees are embedded in the fund returns, not billed separately, which makes them easy to overlook. In a PEO-bundled MEP structure, fund expense ratios can be higher than what employees would access through a standalone plan with a provider like Guideline, Human Interest, or a direct-to-employer Fidelity arrangement. This is a general industry pattern, not a Justworks-specific claim — but it’s worth investigating when you request a plan summary.

To put it plainly: the all-in cost of a PEO-bundled 401(k) is the sum of all three layers. Many business owners compare PEOs on the platform fee alone and don’t account for what the retirement plan actually costs on a per-participant or per-dollar-invested basis. Over time, and especially as employee account balances grow, those embedded fees compound.

The fund lineup itself is curated rather than open-architecture. Justworks and Slavic401k maintain a set menu of investment options — typically a range of index funds, target-date funds, and possibly some actively managed options. For most employees at a small company, a reasonable curated menu is perfectly adequate. But if you have employees who want specific fund access, or if you want to implement a particular investment philosophy in your plan design, you won’t have that flexibility here.

Compare this to a standalone provider like Guideline, which offers transparent flat-fee pricing and open-architecture fund access, or Human Interest, which targets small businesses with low-cost index fund lineups. The tradeoff with those providers is that you’re managing a separate vendor relationship outside your PEO. The tradeoff with Justworks is that you’re paying for convenience, and that convenience has a cost embedded in the plan structure itself.

The right move before committing: ask Justworks for a full fee disclosure document covering all three layers. Any reputable provider will give you this. If the numbers aren’t clear, that’s a signal worth paying attention to.

Employer Match Flexibility and Vesting Schedules

One of the first questions employers ask about any 401(k) is: can I set my own match? And the follow-up: how does vesting work?

Justworks does allow employer contributions, including matching contributions. The degree of customization — whether you can set a fully custom match formula or are limited to predefined tiers — is worth confirming directly with Justworks, as plan design options can evolve. In general, PEO-administered MEPs offer less match formula flexibility than standalone plans, because the plan document has to accommodate multiple employers simultaneously. You’re working within a shared framework, not designing a plan from scratch.

Vesting schedules are a meaningful lever for employee retention. Immediate vesting means employees own 100% of your employer contributions from day one. Graded vesting means they earn ownership over a defined schedule — say, 20% per year over five years. Cliff vesting means they get nothing until a specific tenure threshold, then become fully vested at once.

The choice matters more than most employers realize. If your turnover is high and you’re offering a generous match, immediate vesting is an expensive proposition — you’re effectively giving money to employees who leave after six months. A graded or cliff vesting schedule protects that investment. Conversely, if you’re trying to attract competitive talent in a tight labor market, immediate vesting is a strong selling point. For a team of around 25 people weighing these tradeoffs, our breakdown of Justworks PEO for 25 employees covers the broader cost and fit considerations at that headcount.

Within a PEO-administered MEP, your vesting schedule options are constrained by the plan document. You may have some choices, but you’re not writing the rules from scratch. This is a real limitation compared to sponsoring your own plan.

There’s also an operational reality worth flagging: changing your match formula or vesting schedule mid-year through a PEO is typically more complicated than it sounds. Plan amendments require coordination between Justworks, Slavic401k, and potentially legal review. Some changes can only take effect at the start of a new plan year. If you’re used to the flexibility of making quick HR policy changes, this is a different experience.

This isn’t a reason to avoid Justworks’ retirement plan, but it is a reason to think carefully about your match strategy before you set it up, rather than assuming you can adjust it easily later.

What Happens to the 401(k) When You Leave Justworks

This is the question most business owners don’t ask until they’re already trying to leave. And by then, the answer is more complicated than they expected.

Because your company is participating in Justworks’ MEP rather than sponsoring its own plan, the 401(k) is not portable in the traditional sense. If you leave Justworks — whether you’re switching to a different PEO, moving to a standalone HR platform, or simply outgrowing the service — your employees’ retirement accounts don’t follow you. The plan stays with Justworks and Slavic401k.

What this means practically: when you terminate your Justworks relationship, you’ll need to either roll your employees’ accounts over to a new plan or trigger a plan distribution event. Either way, there’s a transition process that involves paperwork, employee communication, and a blackout period during which employees can’t make changes to their accounts or take distributions.

Blackout periods are a standard part of any 401(k) transition, but they’re disruptive. Employees can’t rebalance portfolios, change contribution rates, or access their accounts during this window. Federal law requires advance notice to participants before a blackout period begins, which adds administrative steps to an already complicated transition.

There’s also the question of continuity. If you’re switching from Justworks to another PEO that has its own 401(k) plan, your employees will need to enroll in the new plan from scratch. Any unvested employer contributions in the Justworks plan are forfeited at that point, depending on your vesting schedule. That’s real money that your employees lose — and it’s a conversation you’ll need to have with your team. Companies that have grown to the point where they’re considering alternatives may want to review how Justworks handles teams of 75 employees to understand where the platform starts to strain.

None of this makes Justworks a bad choice. But it does mean that the switching cost isn’t just the termination fee or the administrative hassle of moving payroll. The retirement plan transition is a genuine friction point, and it should factor into your decision upfront, not as an afterthought when you’re already frustrated and ready to move on.

If there’s any chance you’ll outgrow Justworks within a few years, or if you’re evaluating PEOs as a short-to-medium-term solution rather than a long-term home, the portability limitation of the bundled 401(k) deserves serious weight in your analysis. Reviewing Justworks’ legal history and lawsuits can also help you assess long-term platform stability before making a commitment.

Where This Plan Makes Sense — and Where It Doesn’t

The Justworks 401(k) is a genuinely good fit for a specific type of business. It’s worth being honest about both sides.

Where it works well: Small teams, typically under 50 employees, that don’t currently have a retirement plan and want to offer one without taking on the administrative burden of plan sponsorship. The integrated experience is real — payroll deductions are automatic, enrollment is handled within the Justworks platform, and compliance is largely managed for you. For a founder or operations manager who is already stretched thin, that convenience has genuine value. Our analysis of Justworks for 50 employees explores what changes at that headcount across the full platform, not just retirement benefits.

Where it falls short: Companies that want plan customization, open-architecture fund selection, or a specific investment philosophy won’t find it here. Businesses that anticipate switching PEOs within a few years should factor in the portability friction before committing. And if you already have an existing 401(k) plan with a provider you like, forcing a transition into the Justworks MEP adds disruption without necessarily adding value.

There’s also a middle path worth knowing about. Some businesses use Justworks for payroll and HR administration but maintain a separate 401(k) outside the PEO entirely. This is technically possible, but it creates its own complications. Payroll deductions for the external plan need to be coordinated manually or through a data integration, which adds administrative overhead. You also lose the seamless enrollment experience that makes the bundled plan convenient in the first place. It’s not a clean solution, but for businesses with an established plan they don’t want to disrupt, it’s worth discussing with Justworks before assuming you have to switch. Understanding the Justworks customer support experience can help you gauge how responsive they’ll be during that kind of setup conversation.

One thing worth knowing: the SECURE 2.0 Act (2022) introduced tax credits for small businesses starting new retirement plans — up to $5,000 per year for three years for employers with up to 50 employees. These credits are available whether you use a PEO plan or a standalone plan. If you’re starting a 401(k) for the first time, make sure you’re capturing those credits regardless of which route you take. A tax advisor can confirm eligibility.

The honest summary: Justworks’ retirement offering is a convenience play. It’s not the most cost-efficient option, it’s not the most flexible, and it’s not the most portable. But for small businesses that want a turnkey benefit and don’t want to manage a separate vendor relationship, it does what it’s supposed to do.

The Bottom Line on Justworks 401(k)

The Justworks 401(k) delivers on its core promise: a retirement benefit that’s integrated with your payroll and HR platform, with compliance and administration handled for you. For small businesses that have never offered a retirement plan and want to start without becoming experts in plan governance, that’s a real advantage.

But “convenient” and “optimal” aren’t the same thing. The MEP structure limits customization. The fee layers — platform, administration, and fund-level expense ratios — can add up to more than a standalone plan would cost. And the portability issue is a genuine switching cost that’s easy to underestimate when you’re focused on the monthly per-employee fee.

The right question isn’t just “does Justworks offer a 401(k)?” It’s “what does the all-in cost of this retirement plan look like compared to alternatives, and does the bundled convenience justify that cost for my specific situation?” Those are different questions, and most businesses don’t ask the second one until after they’ve already signed a contract.

Before you renew your PEO agreement or commit to a new one, it’s worth doing that comparison properly. Most businesses overpay on bundled PEO fees because they’re evaluating the headline price rather than the full cost structure. You can compare your options across providers with a side-by-side breakdown of pricing, services, and contract terms — so you’re making a decision based on the complete picture, not just the marketing summary.