Can I contribute to a 401(k) at two different jobs and how do I avoid over-contributing?

You can participate in a 401(k) at each employer, but your personal elective-deferral limit set by the IRS applies to you across all plans combined, not per plan. If you contribute at two jobs, add up your deferrals to make sure the total stays under the annual limit.

This is easy to overshoot when you switch jobs mid-year, because a new employer's payroll system does not know what you already deferred. If you go over (an excess deferral), notify a plan by the deadline and request a corrective distribution of the excess plus earnings, generally before April 15 of the following year, or you face double taxation. Each employer's match, however, is separate and does not reduce your room at the other.