The Romanian Social Democratic Party (Romanian: Partidul Social Democrat Român, or Partidul Social Democrat, PSD) was a social-democratic political party in Romania. In the early 1920s, the Socialist Party of Romania split over the issue of affiliation with the Third International. The majority, which supported affiliation, evolved into the Communist Party of Romania in 1921, while the members who opposed the new orientation formed various political groupings, eventually reorganizing under a central leadership in 1927. From 1938 to 1944, the party was outlawed but remained active in clandestinity. After 1944, it allied with the Communists and eventually reunited with them to form the Romanian Workers' Party in 1948. It published the magazines Socialismul, Lumea Nouă, and Libertatea. After the end of the Communist single-party system in 1989, a group of former members created a new party which proclaimed itself the direct descendent of the PSD.
The first organized party of the Romanian socialists, Romanian Social Democratic Workers' Party was founded in 1893, but was disbanded by the end of the decade after conflicts between the bourgeois leaders, who considered democratic reforms were possible only in alliance with the National Liberal Party, and the proletarian leaders and members, who wanted to continue as a strictly working class party. Lacking material means and organisational experience, the Marxists were only able to re-organise a socialist party in 1910, when the Social Democratic Party of Romania was founded. Outlawed during World War I, the party re-emerged in 1918 with a revolutionary programme, rebranding itself as the Socialist Party of Romania (PSR).