Nonnenwerth is an island in the river Rhine in Germany between Rolandseck and Bad Honnef opposite the island of Grafenwerth.
The island has been the site of a monastery with interruptions since the beginning of the 12th century, was originally founded by the Benedictines and taken over by the Franciscans in 1854. Since then, the monastery has also been home to a Franciscan educational institution, which began as a girls' boarding school and later became a general high school.