The Bulgarian Socialist Party, also known as The Centenarian, is a centre-left, social democratic political party in Bulgaria. The BSP is a member of the Socialist International, Party of European Socialists, and Progressive Alliance. Although founded in 1990 in its modern form, it traces its political heritage back to the founding of the BRSDP in 1891. It is also Bulgaria's largest party by membership numbers.

The Socialist International (SI) is a political international or worldwide organisation of political parties which seek to establish democratic socialism, consisting mostly of social-democratic political parties and labour organisations.