John I of Nassau-Weilburg

John I (1309–1371) was the second son of Count Gerlach I of Nassau. He took power with his elder brother Adolph in 1344, and in the 1355 partition received Weilburg on the Lahn, founding the Nassau-Weilburg line.

In 1356 he fought for France as a mercenary at Poitiers, where his half-brother Kraft was killed and John was captured but quickly released. Emperor Charles IV raised him to an Imperial Prince-Count in 1366. His marriages to Gertrude of Merenberg and Johanna of Saarbrücken brought him Merenberg, Gleiberg and the Saarbrücken inheritance. His son Philip I succeeded him in 1371.