The Movement of Socialist Left (Portuguese: Movimento de Esquerda Socialista, MES) was a Portuguese socialist party, founded shortly after the democratic Carnation Revolution.

The party has its roots in a group of Progressive Christians and Radical Socialists that abandoned the Portuguese Democratic Movement/Democratic Electoral Commissions in 1970, still during the dictatorial regime led by Marcello Caetano.

Among the Party's top figures were César Oliveira, Jorge Sampaio, João Cravinho, Ferro Rodrigues and Augusto Mateus.

The majority of the Party's members would later join the Socialist Party, Jorge Sampaio would become the President of Portugal in 1996 and Ferro Rodrigues would become the leader of the Socialist Party and later Speaker of the Assembly of the Republic.

During its short existence the MES published several newspapers and magazines, such as: Esquerda Socialista, Poder Popular, Acção Sindical, Informação Militante and A Luta Continua.