In government, unicameralism (Latin uni, one + camera, chamber) is the practice of having one legislative or parliamentary chamber. Thus, a unicameral parliament or unicameral legislature is a legislature which consists of one chamber or house. Some countries with unicameral legislatures are often small and homogeneous unitary states and consider an upper house or second chamber unnecessary.

Unicameral legislatures exist when there is no widely perceived need for multicameralism. Many multicameral legislatures were created to give separate voices to different sectors of society. Multiple chambers allowed for guaranteed representation of different social classes (as in the Parliament of the United Kingdom or the French States-General), ethnic or regional interests, or subunits of a federation. Where these factors are unimportant, in unitary states with weak regional identity, unicameralism often prevails. Sometimes, as in New Zealand and Denmark, this comes about through the abolition of one the two chambers, or, as in Sweden, through the merger of the two chambers into a single one, while in others a second chamber has never existed.