Mstislav III Glebovich (before 1215/1220 – after October 18, 1239) was an Olgovichi prince. He was probably prince of Rylsk[1] (1212–1239/1241) and of Chernigov (1235–1239/1241).[1] During his reign, the Tatars (the Mongols) invaded and pillaged the towns of the Principality of Chernigov.[1]