Costantino Lazzari (1 January 1857, Cremona – 29 December 1927, Rome) was an Italian politician.

In 1882 Lazzari founded (with Giuseppe Croce) the Italian Workers' Party (POI).

[2][3] In 1892 with Turati and Anna Kuliscioff, Lazzari founded the Italian Socialist Party (PSI) at the Genoa Congress.

In 1912 he was elected Secretary of the Socialist Party and led the party in the 1913 general election, where the PSI gained 17.6% of votes, arriving second after the governing Liberal Union of Giovanni Giolitti.

After the formation of the Fascist dictatorship of Benito Mussolini, Lazzari was persecuted as a socialist and died in poverty in 1927.