Why is it der Rügener?

Rügener ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] Residents of the island of Rügen

[1] Bewohner der Insel Rügen

Examples

[1] The Rügener Martina and Rügener Adalbert both work in the Tourismus

[1] Die Rügenerin Martina und der Rügener Adalbert arbeiten beide im Tourismus.

Declension

1 Singular Plural
Nominativ der Rügener die Rügener
Genitiv des Rügeners der Rügener
Dativ dem Rügener den Rügenern
Akkusativ den Rügener die Rügener

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary