Leveling seats (Danish: tillægsmandat, Swedish: utjämningsmandat, Norwegian: utjevningsmandater, Icelandic: jöfnunarsæti, German: Ausgleichsmandat), commonly known also as adjustment seats, are an election mechanism employed for many years by all Nordic countries (except Finland) in elections for its national legislatures. Germany also used national leveling seats for its legislature's lower chamber, the Bundestag, from 2013 to 2023. Leveling seats are seats of additional members elected to supplement the members directly elected by each constituency. The purpose of the additional seats is to ensure that each party's share of the total seats is roughly proportional to the party's overall shares of votes at the national level.

In 1915, Denmark became one of the first countries in the world to introduce leveling seats in its parliamentary elections. Since then, all parliamentary elections in Denmark have allocated the adjustment seats as a substantial fraction of the seats in the parliament. The parliamentary seats currently comprise 135 county seats and 40 leveling seats, with a further 4 "North Atlantic seats" elected separately by proportional representation in the Faroe Islands and Greenland (which are not treated as an integral part of the Danish election system). The leveling seats are supplementary to the normal seats, which are allocated by proportional votes within each county. All parties that achieve at least 2% of the national votes are granted as many leveling seats as required to achieve proportional representation at the national level.

Traditionally, Germany's mixed member proportional system awarded overhang seats with no compensation at the national level. That led to a particularly notorious case of negative vote weight after the 2005 German federal election in which voting was delayed by two weeks in the constituency of Dresden I, and it was determined that too much support for Christian Democratic Union would cause the party to lose an overhang seat in the close election. In February 2013, a resultant decision of the Federal Constitutional Court determined negative vote weight was unconstitutional and demanded a reform of the electoral law. The system of leveling seats, which already existed on the state level, was introduced on the national level.

After the 2021 German federal election resulted in a Bundestag with 736 seats, which made it the largest freely-elected parliament in the world because of the large number of leveling seats, debate renewed about how to rein in its size. A 2023 reform removed the leveling seats and instead allocated seats solely by party list votes (meaning that a candidate elected in a local constituency can fail to get into parliament, if their party does not receive enough votes on a national level). That is known as "second vote coverage" (Zweitstimmendeckung).

Leveling seats have been part of the election procedures for all Icelandic parliamentary elections since 1934.

Leveling seats were introduced in Norway in the 1989 parliamentary election, when there were 8 such seats. Since 2005, there are 19 leveling mandates, one for each electoral district. Its current form is based on the following principles:

In the 2005 elections, the average number of votes on a national level was largely similar across party lines. The largest party, the Norwegian Labour Party, required the fewest votes per representative with 14,139; the party that needed the most votes was the Christian People's Party, with 16,262. On a county-by-county basis, however, there were greater disparities: Sogn og Fjordane needed only 3,503 votes to elect one representative from the Liberal Party, but Akershus needed 22,555 to elect one representative from the Socialist Left Party.

The arrangement has gone through several adjustments through the years and is the result of legislative action.