The Belgian Socialist Party (French: Parti Socialiste belge, PSB; Dutch: Belgische Socialistische Partij, BSP) was a social-democratic political party which existed in Belgium from 1945 to 1978.
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 Socialistische Partij for the Flemish community and the Parti Socialiste for the Francophone community.