A regimen is a set of rules or a regulated plan (e.g. of diet or medical treatment) imposed on a person (or commonly, a group of people). The term regimen usually implies a certain strictness by which the treatment plan or the rules are enforced.

Places such as a military institution, prison, convent, school, dormitory, hospital etc. usually expect their inmates to follow a prescribed regimen. The regimen may also include consequences for not following the rules.

In a strict family home, one can also speak of the parent's regimen.