Yes, you can usually put a bonus into your 401(k), and for many people it’s the single most efficient contribution of the year. Two conditions have to line up: your plan document has to treat bonuses as eligible compensation, and you have to change your deferral election before your employer’s payroll cutoff for the bonus cycle. Miss the cutoff and the money is paid as cash; miss the plan-definition check and no election change will route it into the plan at all.
Check Whether Your Plan Treats Bonuses as Eligible Compensation
Federal law lets plans count bonuses toward 401(k) deferrals, but it doesn’t require it. Each plan defines “compensation” in its own document, and some exclude bonuses, commissions, or other supplemental wages.1Internal Revenue Service. 401(k) Plan Fix-It Guide – You Didn’t Use the Plan Definition of Compensation Correctly for All Deferrals and Allocations
Look in your Summary Plan Description or adoption agreement for the definition of “Compensation” and whether bonuses are specifically included or excluded. If you can’t find it, HR or the plan’s third-party administrator can confirm in a sentence. Do this before you spend time adjusting anything, because if the plan excludes bonuses, no election gets that money into your account.
How to Change Your Contribution Before Bonus Day
Timing is the whole game. Your election has to be in before payroll processes the bonus. Most administrators require changes several business days ahead, and some lock elections a full pay period in advance. Once the bonus enters processing, it’s too late.
Log in to your plan’s administrator portal or ask HR for the correct form, sometimes called a Salary Reduction Agreement. Many payroll systems accept only percentages for bonus deferrals, not flat dollar amounts. If you want the entire bonus to go in, set the rate to 100% of supplemental pay. FICA will still be withheld from the gross before the deferral is applied, so the net going into the plan will be less than the headline number.
Watch for a separate field labeled “Bonus Election” or “Supplemental Pay,” distinct from your regular salary deferral. Using the wrong field is the most common mistake. Change your regular deferral by accident and every future paycheck is affected until you change it back. If your portal has no separate field, the plan is probably applying your regular deferral percentage to bonuses too. Save the confirmation email or number after you submit; if payroll gets it wrong, that’s your proof the election was made on time.
What Deferral Saves You in Taxes, and What It Doesn’t
A bonus paid as cash is treated as supplemental wages. For 2026, federal income tax is withheld at a flat 22% on supplemental wages, and at 37% on any portion above $1 million for the year.2Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide Deferring the bonus into a traditional pre-tax 401(k) avoids that income tax withholding on the deferred portion.
Social Security and Medicare taxes are a different story. Federal law treats elective deferrals as wages for FICA, so payroll takes 6.2% for Social Security (up to the wage base) and 1.45% for Medicare on the full bonus even if you defer every dollar.3Office of the Law Revision Counsel. 26 USC 3121 – Definitions The savings come from income tax, not payroll tax. State income tax withholding on supplemental wages is usually reduced by a pre-tax deferral as well, though the rules vary by state.
If your plan offers a designated Roth account, you can send the bonus there instead. You pay income tax on the bonus now, but qualified withdrawals in retirement come out tax-free, growth included.4Internal Revenue Service. Roth Account in Your Retirement Plan A bonus is a reasonable candidate for a Roth deferral if you expect your tax rate to be higher later, since the 22% withholding would have applied anyway. If you’re in a high-earning year and expect lower income in retirement, the traditional side gives you a bigger benefit today.
The Employer Match Trap
Deferring a large bonus early in the year can quietly cost you employer matching money. Most plans calculate the match per paycheck. If your employer matches 50% of the first 6% you contribute each pay period, and a big bonus deferral in March pushes you to the annual deferral limit, you stop contributing for the remaining nine months. No employee contribution, no match, for three quarters of the year.
The fix is a true-up provision. A plan with a true-up recalculates the match at year-end based on total annual pay and total annual deferrals, then adds whatever match you would have earned if you’d contributed evenly. Not every plan has one. Before routing a large bonus into your 401(k), ask HR two questions: is the match calculated per paycheck or annually, and if per paycheck, is there a true-up? If both answers point the wrong way, spreading contributions across the year usually beats front-loading through a bonus.
2026 Contribution Limits to Keep in View
The IRS caps elective deferrals under Internal Revenue Code Section 402(g). For 2026:
- Under age 50: $24,500
- Age 50 and older: $24,500 plus an $8,000 catch-up, for $32,500
- Ages 60 through 63: $24,500 plus an $11,250 “super catch-up” under SECURE 2.0, for $35,750
The super catch-up for ages 60 through 63 replaces the standard catch-up during those four years, then reverts to the regular $8,000 at age 64.5Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 These limits apply across all 401(k) plans you participate in during the year, not per plan. If you changed jobs and deferred at both employers, the combined total still cannot exceed your cap.6Office of the Law Revision Counsel. 26 USC 402 – Taxability of Beneficiary of Employees’ Trust The same cap applies whether your deferrals are traditional, Roth, or a mix.
Check the Pay Stub, and Fix an Over-Contribution Fast
When the bonus stub lands, confirm the 401(k) deduction matches what you elected, verify that FICA came out of the gross (it should), and look at your year-to-date deferral total against your applicable limit. If you temporarily set a special rate for the bonus, switch your regular deferral back to your normal percentage before the next paycheck.
If a bonus deferral pushes you past the 402(g) limit, the excess is included in your taxable income for the year you contributed it. Leave it in the plan and you’ll be taxed on that money twice: once now and again when you withdraw it in retirement.7Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan
To avoid the second layer of tax, notify the plan and have the excess plus any earnings distributed back to you by April 15 of the year after the over-contribution. For excess deferrals made in 2026, that deadline is April 15, 2027, and it does not move if you file a tax extension.6Office of the Law Revision Counsel. 26 USC 402 – Taxability of Beneficiary of Employees’ Trust Most single-employer payroll systems stop deferrals automatically at the limit. The situation to watch is two employers in the same year, where neither system sees the other’s totals; in that case, contacting one of the plans in writing before April 15 is on you.