Mstislav III Glebovich (before 1215/1220 – after October 18, 1239) was an Olgovichi prince. He was probably prince of Rylsk (1212–1239/1241) and of Chernigov (1235–1239/1241). During his reign, the Tatars (the Mongols) invaded and pillaged the towns of the Principality of Chernigov.
He was the son of Prince Gleb Svyatoslavich of Chernigov and Anastasia Ryurikovna, a daughter of Grand Prince Ryurik Rostislavich of Kiev. His father died between 1215 and 1220.
By 1225, Mstislav had already been second in seniority among the Olgovichi (the ruling dynasty of Chernigov), and therefore during the absence of his cousin, Mikhail Vsevolodovich, he commanded them. On April 6, 1231, he attended a snem (a meeting of some leading princes of Rus’ organized by Grand Prince Vladimir III of Kiev) in Kiev, but the reasons for convoking the council are not given. It appears that his domain probably lay west of the Snov and Desna rivers.