The Riksdag (Swedish: [ˈrɪ̌ksdɑːɡ] ^ⓘ or [ˈrɪ̌ksda], lit. '"diet of the realm"'; also Swedish: riksdagen [ˈrɪ̌ksdan] ^ⓘ or Sveriges riksdag, pronounced [ˈsvæ̌rjɛs ˈrɪ̌ksdɑːɡ] or [- ˈrɪ̌ksda] ^ⓘ) is the parliament and the supreme decision-making body of the Kingdom of Sweden. Since 1971, the Riksdag has been a unicameral parliament with 349 members (riksdagsledamöter), elected proportionally and serving, since 1994, fixed four-year terms. The 2022 Swedish general election is the most recent general election.

The constitutional mandates of the Riksdag are enumerated in the Instrument of Government (Regeringsformen), and its internal workings are specified in greater detail in the Riksdag Act (Riksdagsordningen). The seat of the Riksdag is at Parliament House (Riksdagshuset), on the island of Helgeandsholmen in central Stockholm, in Gamla stan, the old town of Stockholm. The Riksdag has its institutional roots in the feudal Riksdag of the Estates, traditionally thought to have first assembled in Arboga in 1435. In 1866, following reforms of the 1809 Instrument of Government, that body was transformed into a bicameral legislature with an upper chamber (första kammaren) and a lower chamber (andra kammaren).

The Swedish word riksdag, in definite form riksdagen, is a general term for "parliament" or "assembly", but it is typically only used for Sweden's legislature and certain related institutions. In addition to Sweden's parliament, it is also used for the Parliament of Finland and the Estonian Riigikogu, as well as the historical German Reichstag and the Danish Rigsdagen. In Swedish use, riksdagen is usually not capitalised. Riksdag derives from the genitive of rike, referring to royal power, and dag, meaning diet or conference; the German word Reichstag and the Danish Rigsdag are cognate. The Oxford English Dictionary traces English use of the term "Riksdag" in reference to the Swedish assembly back to 1855.

The roots of the modern Riksdag can be found in a 1435 meeting in the city of Arboga; however, only three of the estates were probably present: the nobility, the clergy and the burghers. This informal organization was modified in 1527 by the first modern Swedish king, Gustav I Vasa, to include representatives from all the four social estates: the nobility, the clergy, the burghers (property-owning commoners in the towns such as merchants etc.), and the yeomanry (freehold farmers). This form of Ständestaat representation lasted until 1866, when representation by estate was abolished and the modern bicameral parliament established. Effectively, however, it did not become a parliament in the modern sense until parliamentary principles were established in the political system in Sweden, in 1917.

On 22 June 1866, the Riksdag decided to reconstitute itself as a bicameral legislature, consisting of Första kammaren or the First Chamber, with 155 members and Andra kammaren or the Second Chamber with 233 members. The First Chamber was indirectly elected by county and city councillors, while the Second Chamber was directly elected by universal suffrage. This reform was a result of great discontent with the old Estates, which, following the changes brought by the beginnings of the Industrial Revolution, was no longer able to provide representation for large segments of the population.

By an amendment to the 1809 Instrument of Government, the general election of 1970 was the first to a unicameral assembly with 350 seats. The following general election to the unicameral Riksdag in 1973 gave the Government the support of only 175 members, while the opposition could mobilize an equal force of 175 members. In a number of cases a tied vote ensued, and the final decision had to be determined by lot. To avoid any recurrence of this unstable situation, the number of seats in the Riksdag was reduced to 349, from 1976 onwards.

The Riksdag performs the normal functions of a legislature in a parliamentary democracy. It enacts laws, amends the constitution and appoints a government. In most parliamentary democracies, the head of state commissions a politician to form a government. Under the new Instrument of Government (one of the four fundamental laws of the Constitution) enacted in 1974, that task was removed from the Monarch of Sweden and given to the Speaker of the Riksdag. To make changes to the Constitution under the new Instrument of Government, amendments must be approved twice, in two successive electoral periods with a regular general election held in between.

There are 15 parliamentary committees in the Riksdag.