John III of Mecklenburg

John III (after 1266–1289) was a son of Henry I of Mecklenburg and Anastasia of Pomerania. From 1287 he ruled Mecklenburg jointly with his elder brother Henry II, while their uncle Nicholas III acted as regent. On 3 November 1288 he married Helena, daughter of Wizlaw II of Rügen.

His rule was short. On 27 May 1289 he drowned off the island of Poel and was buried in the Franciscan monastery at Wismar.