The Social Democratic Party of Switzerland (German: Sozialdemokratische Partei der Schweiz, SP; Romansh: Partida Socialdemocrata de la Svizra), also rendered as the Swiss Socialist Party (French: Parti socialiste suisse, PS; Italian: Partito Socialista Svizzero), is a political party in Switzerland. The SP has had two representatives on the Swiss Federal Council since 1960 and received the second highest total number of votes in the 2019 Swiss federal election.

The SP was founded on 21 October 1888 and is currently the second largest of the four leading coalition political parties in Switzerland. It is the only party on the left with representatives on the Swiss Federal Council, currently Alain Berset and Simonetta Sommaruga. As of September 2019, the SP is the second largest political party in the Swiss parliament.

Unlike most other Swiss parties, the SP is the largest pro-European party in Switzerland and supports Swiss membership of the European Union. Additionally, it is strongly opposed to capitalism and maintains a long-term goal of "overcoming capitalism". The party is a member of the Progressive Alliance and an associate member of the Party of European Socialists.