Why is it die Wienerin?

Wienerin ends in -in, and most nouns ending in -in are feminine: in a German dictionary, 88% of them take die.

All nouns ending in -in
Sense 1
die

Meaning

[1] inhabitant of the city of Vienna

[1] Einwohnerin der Stadt Wien

Examples

[1] She is Wieninä

[1] Sie ist Wienerin.

Declension

1 Singular Plural
Nominativ die Wienerin die Wienerinnen
Genitiv der Wienerin der Wienerinnen
Dativ der Wienerin den Wienerinnen
Akkusativ die Wienerin die Wienerinnen

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary