Generally conditional — permissible as a structure, but fund selection determines the actual ruling.
Confidence level: Strong on the general framework. The specific ruling for your account depends on which funds your plan actually offers.
THE CORE RULING
A 401(k) is a tax-deferred retirement account created by US law — it is a container, not a contract of debt or interest in itself. Contributing to one, and even receiving an employer match, is not inherently impermissible. What determines the ruling is where the money inside the account is actually invested. Most default 401(k) options — target-date funds, S&P 500 index funds, total bond market funds — hold a mix of conventional bonds (which pay interest) and shares in companies that fail standard Shariah screens (conventional banks, insurers, highly leveraged firms, and businesses with material income from alcohol, gambling, or interest).
WHY THE ACCOUNT ITSELF ISN'T THE PROBLEM
Three things about a 401(k) are neutral from a Shariah standpoint: the tax deferral (a legal/tax mechanism, not a contract), the employer match (a form of compensation, not a loan), and mandatory required minimum distributions later in life (a withdrawal rule, not a financial transaction). None of these involve you lending or borrowing money at interest. The riba concern is entirely downstream — it lives in the underlying holdings of whichever funds you pick.
WHERE THE ACTUAL RISK IS
Bond funds and "stable value" funds pay a return that is structurally interest — these should be avoided entirely where alternatives exist. Broad index and target-date funds hold a mix of compliant and non-compliant equities; the non-compliant portion (roughly estimated by standard screens) is the part that needs purifying. Company stock or sector funds should be checked individually — heavy debt load or interest income are the two most common disqualifiers.
WHAT YOU SHOULD DO
If your plan has a Shariah-compliant fund option (rare but growing) or a Self-Directed Brokerage Account (SDBA) window: use it to invest in screened funds (e.g., Amana Mutual Funds, Wahed, or Shariah ETFs) or individually screened stocks. If your plan only offers conventional funds: take the employer match — it is close to free compensation and declining it doesn't remove any riba, it just forfeits money — then choose the fund with the least debt/interest exposure (typically a growth or equity fund over a bond fund), and separately purify the estimated impermissible portion of any gains, usually by donating an amount proportional to the non-compliant income (many screening services publish this "purification ratio" per stock or fund).
WHAT COULD CHANGE THIS ANSWER
— Your plan offers a genuine Shariah-compliant fund or an SDBA — The specific funds available to you are more or less compliant than a typical default lineup — You have other retirement savings options (e.g., an IRA) where full Shariah screening is possible — A qualified scholar reviews your specific plan's fund menu and purification needs
This confirms the general framework only. It is not a personal fatwa, tax advice, or investment advice. Review your specific plan's fund options, ideally with a screening service or qualified scholar, before finalizing your allocation.