Lydia Osipovna Dan (Russian: ??´??? ?´??????? ???; 21 May 1878 – 28 March 1963) was a Menshevik revolutionary leader and active participant in the revolutionary movement of Russia.
Her brother was the Menshevik leader Julius Martov, her husband was fellow Menshevik Fyodor Dan, and she was the godmother of Yuri Larin.