The Leftwing Union for the Socialist Democracy was a Portuguese leftwing party founded in January of 1978. The party has its origins in the Socialist Culture Association - Worker Brotherhood, a socialist organization and in groups of independent people linked to the Socialist Party.
The party participated in the legislative election of 1980 in coalition with the Socialist Party and the Independent Social Democratic Action in the Republican and Socialist Front. In the subsequent elections, the party's members integrated the lists of the Socialist Party. In the Presidential election of 1986 the UEDS members split, one part supporting Mário Soares and the other supporting Maria de Lurdes Pintasilgo.
The party was disbanded in 1986 and some of its members joined the Socialist Party.