Ion Costache Frimu (October 16 [O.S. October 4] 1871 – February 19 [O.S. February 6] 1919) was a Romanian socialist militant and politician, a leading member of the Social Democratic Party of Romania (PSDR) and . He died after being beaten and contracting an illness in prison, where he was being held for his participation in the typographic workers' demonstration of December 1918.