The Belgian Socialist Party (French: Parti Socialiste belge, pronounced [paʁti sɔsjalist bɛlʒ], PSB; Dutch: Belgische Socialistische Partij, pronounced [ˈbɛlɣisə ˌsoːɕaːˈlɪstisə pɑrˈtɛi], BSP) was a social-democratic political party which existed in Belgium from 1945 to 1978. During its time in office, a number of progressive social reforms were introduced.

The BSP was founded by activists from the Belgian Labour Party (1885–1940), which was the first Belgian socialist party. It ceased to function during the Second World War, while Belgium was under Nazi occupation. Its main support bases were the co-operative and trade union movements, and it won relatively more support in Wallonia. Like most Belgian political organisations, the party supported greater integration with the European Economic Community, albeit in a socialist context.

As linguistic and community issues became more divisive, the Belgian Socialist Party split into two new entities: the Flemish Socialist Party for the Flemish community and the Parti Socialiste (PS) for the Francophone community.

^a From the 1971 general election, the Belgian Socialist Party ran separate lists for Flanders and Wallonia; however, they still existed under a single party. The letters in bold thus show the results of the combined lists and consequently the true result of the Belgian Socialist Party in each election
^b Whilst the Belgian Socialist Party also ran separate lists for Flanders and Wallonia in the 1974 general election, there is no information on the results of separate lists, hence only the result for the combined lists is shown.

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