Mstislav I Vladimirovich the Great (Мстислав Владимирович Великий in Russian) (June 1, 1076 — April 14, 1132), Grand Prince of Kiev (1125-1132), the eldest son of Vladimir Monomakh by Gytha of Wessex. He figures prominently in the Norse Sagas under the name Harald, taken to allude to his grandfather, Harold II of England.

St Nicholas Cathedral, built by Mstislav I near his palace at Yaroslav's Court, Novgorod, contains 12th-century frescoes depicting his illustrious family

As his father's future successor, Mstislav reigned in Novgorod the Great from 1088-93 and (after a brief stint at Rostov) from 1095-1117. Thereafter he was Monomakh's co-ruler in Belgorod-on-the-Dnieper, and inherited the Kievan throne after his death. He built numerous churches in Novgorod, of which St Nicholas Cathedral (1113) and the cathedral of St Anthony Cloister (1117) survive to the present day. Later, he would also erect important churches in Kiev, notably his family sepulchre at Berestovo and the church of Our Lady at Podil.

Mstislav's life was spent in constant warfare with Cumans (1093, 1107, 1111, 1129), Estonians (1111, 1113, 1116, 1130), Lithuanians (1131), and the princedom of Polotsk (1127, 1129). In 1096, he defeated his uncle Oleg of Chernigov on the Koloksha River, thereby laying foundation for the centuries of enmity between his and Oleg's descendants. Mstislav was the last ruler of united Rus, and upon his death, as the chronicler put it, "the land of Rus was torn apart".

By his wife, Christine of Sweden (+1122), Mstislav had many children, of which Izyaslav II founded the House of Galicia, and Rostislav founded the House of Smolensk-Yaroslavl. Of his daughters, Ingeborg married king Canute Lavard of Jutland and was the mother of Valdemar the Great, Malfrid married king Sigurd I of Norway and then Eric II of Denmark, Euphrosyne married king Geza II of Hungary, and Eupraxia married Byzantine Emperor Alexius I Comnenus.