Otto Ostrowski (28 January 1883 in Spremberg – 16 June 1963 in Knokke, Belgium) was a German politician of the Social Democratic Party of Germany (SPD) and Mayor of Berlin in 1946–1947.

Ostrowski graduated in Romance studies and after World War I joined the SPD.

A fierce opponent of the Nazis, he helped hiding Jewish persecutees to protect them from deportation to the extermination camps.

He stepped into office on 8 January 1947, but from the beginning had to deal with the uncertain relationship to the deputies of the Socialist Unity Party of Germany (SED).

Though the SPD in the western sectors – including Ostrowski himself – refused this union, he appointed SED members for the city council against strong resistance in his party and was finally voted out of office on 11 April 1947.