You can contribute to a 401(k) for the previous year, but only the employer side. Your own salary deferrals from your paycheck are locked to the calendar year they came out of, with a hard December 31 cutoff and no extensions. Employer contributions such as matching and profit-sharing work differently: they can be deposited well into the following year and still count for the prior tax year, as late as the business’s tax-return due date including extensions. That single split between employee money and employer money answers most of the question.
Why Your Own Deferrals Cannot Be Made Retroactively
The money you personally direct into a 401(k) comes out of your paychecks during the year. A deferral you wanted counted for 2025 had to be withheld from 2025 wages by December 31, 2025. There is no mechanism to write a check in February and label it a prior-year contribution the way you can with an IRA.
This restriction comes from a concept called constructive receipt. Once you could have received the wages but chose to redirect them into the plan, the deferral is fixed to that tax year. The amount appears on your W-2 in Box 12 for the year it was withheld, and the annual deferral limit is measured on a calendar-year basis.1eCFR. 26 CFR 1.402(g)-1 – Limitation on Exclusion for Elective Deferrals If you didn’t use your full deferral capacity by December 31, that capacity is gone. You cannot recapture it in the new year.
The same calendar-year cutoff applies to designated Roth 401(k) contributions. Pre-tax and Roth deferrals share the same annual limit and the same December 31 deadline.2Internal Revenue Service. Retirement Plans FAQs on Designated Roth Accounts For 2026, the employee deferral limit is $24,500. If you’re 50 or older, you can add $8,000 in catch-up. Participants who turn 60, 61, 62, or 63 during the year get an enhanced catch-up of $11,250 under a SECURE 2.0 provision.3Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Whatever your ceiling, if you don’t hit it by December 31, that year’s opportunity closes.
Employer Contributions Can Be Deposited After Year-End
The employer side of a 401(k) is where prior-year contributions genuinely happen. Under federal tax law, an employer contribution is treated as if it were made on the last day of the preceding year, provided the money reaches the plan by the due date of the employer’s tax return, including extensions.4Office of the Law Revision Counsel. 26 USC 404 – Deduction for Contributions of an Employer to an Employees Trust or Annuity Plan Both matching contributions and discretionary profit-sharing contributions qualify.5Internal Revenue Service. Issue Snapshot – Deductibility of Employer Contributions to a 401(k) Plan Made After the End of the Tax Year
The exact deadline depends on the business’s structure:
- C-corporations filing Form 1120: April 15, with a six-month extension to October 15.
- S-corporations filing Form 1120-S and partnerships filing Form 1065: March 15, with a six-month extension to September 15.
- Sole proprietors reporting on Schedule C with Form 1040: April 15, with a six-month extension to October 15.
A company that files an extension and deposits a profit-sharing contribution in September is making a valid prior-year contribution. The plan document has to authorize the contribution, and the employer needs to treat it as allocated to the prior year for plan purposes even though the cash arrives later.5Internal Revenue Service. Issue Snapshot – Deductibility of Employer Contributions to a 401(k) Plan Made After the End of the Tax Year
When you stack employee deferrals and employer contributions together, total additions for 2026 cannot exceed $72,000 per person, or $80,000 with the standard catch-up and $83,250 with the enhanced catch-up.6Internal Revenue Service. Notice 2025-67 – 2026 Amounts Relating to Retirement Plans and IRAs The combined cap matters if you’re relying on a large employer contribution to make up for a small deferral.
Solo 401(k): Two Deadlines Inside One Plan
A Solo 401(k), designed for owner-only businesses with no full-time employees other than the owner and their spouse, is where the two-deadline structure gets practical. The owner wears both hats, so the plan has two contribution buckets, each with its own deadline. This is where self-employed savers most often get confused.
The employee deferral portion still follows the December 31 rule. If you wanted to defer $24,500 of your 2025 self-employment income, that election had to be made and the money set aside by December 31, 2025. Missing that date closes the deferral for the year.
The employer profit-sharing portion follows the tax-return deadline. A sole proprietor filing Schedule C can deposit the employer contribution by April 15, or by October 15 with an extension. This is genuinely useful, because you can wait until you’ve calculated your actual net self-employment income, then contribute up to 25% of net earnings (after the self-employment tax deduction) all the way up to the combined cap.5Internal Revenue Service. Issue Snapshot – Deductibility of Employer Contributions to a 401(k) Plan Made After the End of the Tax Year
The SECURE 2.0 First-Year Exception
One rule catches many self-employed people by surprise. For plan years beginning after December 29, 2022, SECURE 2.0 created a one-time exception to the December 31 deferral deadline for sole proprietors and single-member LLCs setting up a Solo 401(k) for the first time. Under this provision, the owner can establish the plan and make retroactive employee deferrals for the initial plan year, as long as everything is funded by the individual tax-return due date.
One important detail: this deadline does not include extensions. If your Form 1040 is due April 15, that is the cutoff for the first-year retroactive deferral, even if you file an extension. The exception applies only to the first year of the plan’s existence, and only to sole proprietors and single-member LLCs. After year one, the standard December 31 deferral deadline returns. It’s a real tool for someone who had a profitable year but didn’t think about opening a Solo 401(k) until January or February.
Why This Feels Different From an IRA
Most of the confusion around prior-year 401(k) contributions comes from the fact that IRAs work the opposite way. For a Traditional or Roth IRA, you can make contributions for the 2025 tax year any time up to April 15, 2026. The IRA contribution limit for 2026 is $7,500, or $8,500 if you’re 50 or older.3Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 You tell the custodian which tax year the deposit counts toward, and that’s it. No payroll system is involved and no employer needs to act.7Internal Revenue Service. IRA Year-End Reminders
SEP-IRAs go further still. Because a SEP-IRA is funded entirely by employer contributions, the entire contribution can be made up to the business’s tax-filing deadline including extensions.8Internal Revenue Service. Retirement Plans FAQs Regarding SEPs A sole proprietor who files an extension has until October 15 to fund a SEP-IRA for the prior year. There’s no employee deferral component in a SEP, so the December 31 issue never comes up. The trade-off is that a SEP-IRA lacks the employee deferral bucket entirely, which limits total contributions for owners with lower net income. That’s why the Solo 401(k) often wins for the self-employed despite the split deadline: the deferral lets you shelter income regardless of profits, while the profit-sharing piece scales with earnings.
What If You Already Contributed Too Much
If your total salary deferrals across all plans exceed the annual limit, the excess has to come out of the plan by April 15 of the following year. Withdraw the excess by that deadline and the amount is taxed as income in the year it was originally deferred, with any earnings on the excess taxed in the year distributed.9Internal Revenue Service. 401(k) Plan Fix-It Guide – Elective Deferrals Were Not Limited to the Amounts Under IRC Section 402(g)
Miss the April 15 deadline and the excess gets taxed twice: once in the year deferred, again when it’s eventually distributed from the plan. You also lose the ability to claim basis in the excess, so there’s no credit for the taxes you already paid on it.10Internal Revenue Service. Consequences to a Participant Who Makes Excess Annual Salary Deferrals The people most often affected are those who changed jobs mid-year and deferred into two separate plans without coordinating the totals. The annual deferral limit applies per person across all 401(k)-type plans, not per plan. If you contact the plan administrator early in the new year, the correction is straightforward. Wait too long and it stops being fixable.