The additional member system (AMS) is a mixed electoral system under which most representatives are elected in single-member districts (SMDs), and the other "additional members" are elected to make the seat distribution in the chamber more proportional to the way votes are cast for party lists. It is distinct from parallel voting (also known as the supplementary member system) in that the "additional member" seats are awarded to parties taking into account seats won in SMDs (referred to as compensation or "top-up"), which is not done under parallel voting (a non-compensatory method).

AMS is categorised under semi-proportional electoral systems, differently from the mixed-member proportional representation (MMP). In practice, the way that proportional additional member systems work depends on the number of additional ("top-up") seats and the votes cast in a specific election.

This article focuses primarily on semi-proportional implementations of AMS, like the ones used in the UK. AMS is used by the Scottish Parliament, the Welsh Parliament and the London Assembly. In Scotland and Wales list members ("top-up" seats) are elected by region; in London there is a single London-wide pooling of list votes.