John III of Holstein-Plön

John III, called the Mild (c. 1297–1359), was a count of Holstein-Plön and Holstein-Kiel, son of Gerhard II the Blind and Agnes of Brandenburg, and half-brother of Kings Eric VI and Christopher II. In 1312 he succeeded in Plön and inherited Kiel in 1316.

A chief creditor of Denmark, from 1332 to 1340 he and Count Gerhard III of Rendsburg guarded Schleswig. He died in 1359 and was succeeded by his son Adolphus.