A 401(k) is a type of retirement savings account in the United States, which takes its name from subsection 401(k) of the Internal Revenue Code (Title 26 of the United States Code). A contributor can begin to withdraw funds after reaching the age of 59½ years. (See subsection "Withdrawal of funds" below for restrictions before that age.) 401(k)s were first widely adopted as retirement plans for American workers, beginning in the 1980s. The 401(k) emerged as an alternative to the traditional retirement pension, which was paid by employers. Employer contributions with the 401(k) can vary, but in general the 401(k) had the effect of shifting the burden for retirement savings to workers themselves. In 2011, about 60% of American households nearing retirement age had 401(k)-type accounts.

The first cohort of workers to widely adopt this style of retirement plan are beginning to retire, and the plans now appear to generally be falling short; this shortfall, however, does not appear to be attributable to the mechanics of the system itself but rather the employees not taking advantage of the benefits provided by it. That is, as with a defined benefit plan employees are not required to do anything in order to obtain benefits, a 401(k) plan calls for the employee to allocate portions of his/her income towards retirement (as opposed to taking it home and spending currently). According to a Feb 19, 2011 article in the Wall Street Journal, "the median household headed by a person aged 60 to 62 with a 401(k) account has less than one-quarter of what is needed in that account to maintain its standard of living in retirement." This according to a study commissioned by the Journal, and conducted by the Center for Retirement Research at Boston College.

Employers can help their employees save for retirement while reducing taxable income under this provision, and workers can choose to deposit part of their earnings into a 401(k) account and not pay income tax on it until the money is later withdrawn in retirement. Interest earned on money in a 401(k) account is never taxed before funds are withdrawn. Employers may choose to, and often do, match contributions that workers make. The 401(k) account is typically administered by the employer, while in the usual "participant-directed" plan, the employee may select from different kinds of investment options. Employees choose where their savings will be invested, usually, between a selection of mutual funds that emphasize stocks, bonds, money market investments, or some mix of the above. Many companies' 401(k) plans also offer the option to purchase the company's stock. The employee can generally re-allocate money among these investment choices at any time. In the less common trustee-directed 401(k) plans, the employer appoints trustees who decide how the plan's assets will be invested.

Since 2006, another type of 401(k) plan has been available. Participants in 401(k) plans that have the proper amendments can allocate some or all of their contributions to a separately-designated Roth account, commonly known as a Roth 401(k). These "Roth" contributions will be collected and treated as after-tax dollars; that is, income tax is paid or withheld in the year contributed. Qualified distributions from a designated Roth 401(k) account, including all income, are tax-free. (A traditional 401(k) account is funded with pre-tax dollars and, in general, tax must be paid when the original contribution and earnings are withdrawn.) All employer matching funds are deposited into the account on a pretax basis, even if the employee's contributions are all Roth contributions. Employer contributions may be subject to vesting rules set by the plan documents requiring the employee to reach a certain number of years of service before they are entitled to keep the matching funds.

For non-profit organizations the corresponding plan is found in 26 USC 403(b) and for government entities it is 26 U.S.C. § 457, although older plans were established under 457(g).

Initialism

IRA

1. Irish Republican Army
2. (US) Individual Retirement Account
3. Investment Recovery Association

(Irish Republican Army):

• Continuity IRA
• Old IRA
• Official IRA
• Provisional IRA
• PIRA
• Real IRA

(individual retirement account):

• Roth IRA

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