The answer to this question depends on the type of retirement plan.

Generally, if your employer's plan has a separate account for each employee, it is a defined contribution plan.

In the other type of plan, a defined benefit plan:

The Form W-2 (PDF) you receive from your employer has a box used to indicate whether you were covered for the year. The "Retirement Plan" box should be checked if you were covered in a plan sponsored by the employer.

If you receive retirement benefits in the form of pension or annuity payments, the amounts you receive may be fully taxable, or partly taxable in the year received.

Generally, your pension or annuity is usually fully taxable:

Generally, your pension or annuity will be partially taxable:

If you receive pension or annuity payments before age 59-1/2, you may be subject to an additional 10% tax on early distributions.  See Publication 575.

Note: If you contributed after-tax dollars in the form of designated Roth contributions to a 401(k) plan that permits such contributions, these contributions would be fully taxable in the year of contributions, although qualified distributions from the designated Roth account would not be taxed when received.

The rules for retirement plans are complex. Your plan administrator should have written information about your particular plan that explains the limitations imposed by law as well as other limitations that apply under the plan:

A lump-sum distribution is the distribution or payment, within a single tax year, of an employee's entire balance from all of the employer's qualified pension, profit-sharing, or stock bonus plans. If you were born on or before January 1, 1936, or are the beneficiary of a participant born on or before January 1, 1936,  you may be able to able to elect optional methods of figuring the tax on lump-sum distributions you received from an eligible retirement plan.  These optional methods can be elected only once after 1986.

For other situations and further information, see Publication 575, Pension and Annuity Income.

 

 

If the amount rolled over was the net amount (the amount of the distribution less the tax withheld):

If the amount rolled over was the gross amount (the amount rolled over included the 20% that was withheld):

 

Generally, unless an exception applies, a distribution of your benefits from a 401(k) plan before age 59 ½ is subject to the 10% additional tax on early distributions from retirement plans. See Publication 575, Pension and Annuity Income.

 

However, there are special rules that apply to earlier distributions.  For example, a distribution from a 401(k) plan that is made after separation from service and age and in in each year after you reach 55 is not subject to the 10% tax.

If you default on a loan from your 401(k) plan:

For example, the 10% additional tax on early distributions does not apply if all the following apply to you:

There are a number of exceptions to the 10% additional tax on early distributions. You may wish to refer to Instructions for Form 5329, Additional Taxes on Qualified Plans (Including IRAs) and Other Tax-Favored Accounts Publication 575, Pension and Annuity Income, and Publication 560, Retirement Plans for Small Business, for additional information.