Masław (also Miecław, Mojsław and Miesław, ?-1047) was a cup-bearer of Mieszko II of Poland and a revolter who tried to establish his own state in Masovia.

Little is known about the early life of Masław. Initially he was a cup-bearer of Mieszko II and his governor of Masovia. However, the regional crisis that led to the rule of Bezprym, weakened the overlordship over Masovia.

Following the death of Mieszko II in 1034, Poland experienced a period of revolts and massive anti-Christian reaction. Initially Casimir I could not gain enough power to reunite the country and the region succumbed into chaos. The pagan reaction in Masovia was particularily strong and it is assumed that Masław simply took advantage of the difficult situation to create a state of his own in the lands he controlled under the reign of Mieszko. However, in 1039 Casimir I was supported by Henry III, Holy Roman Emperor against the Bohemians and soon afterwards the legitimate ruler also concluded an alliance with Kievan Ruthenia. This allowed him to gather enough forces to attack the revolted province. Also, Masovia was under constant attacks by pagan Pomeranians, Prussians and Jaćwież. Probably these attacks did not allow Masław to successfully defend his newly-born state.

Finally in 1047 he was attacked by joint forces of Casimir I the Restorer and Yaroslav I the Wise and was probably killed in a battle at the shores of Vistula.