United Left (Italian: Sinistra Unita, SU) was a left-wing political party in San Marino.

In the Sammarinese parliamentary election, 2006, the alliance won 8.67% of the votes cast and took five seats.

The United Left was part of the governing coalition that governed San Marino from 2006 to 2008 along with the Party of Socialists and Democrats and the Popular Alliance of Sammarinese Democrats for the Republic until tensions between itself and the latter caused the coalition to disintegrate.

For the 2008 general election United Left was part of the Reforms and Freedom electoral coalition which won 25 seats out of 60 in the Grand and General Council gaining 45.78% of the national vote.

During the election of 2012, SU created a leftist alliance with Civic 10 called Active Citizenship which obtained the 16% of votes.