Why is it der Indonesier?

Indonesier 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] Citizens of Indonesia

[1] Staatsbürger von Indonesien

Examples

[1] In Jakarta, the capital of Indonesia, over nine million Indonesians live

[1] In Jakarta, der Hauptstadt Indonesiens, leben über neun Millionen Indonesier.

Declension

1 Singular Plural
Nominativ der Indonesier die Indonesier
Genitiv des Indonesiers der Indonesier
Dativ dem Indonesier den Indonesiern
Akkusativ den Indonesier die Indonesier

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary