Why is it der Extremismus?

Extremismus ends in -ismus, and nouns ending in -ismus are masculine: in a German dictionary, 100% of them take der.

All nouns ending in -ismus
Sense 1
der

Meaning

[1] mostly singular: extreme political position, attitude, direction

[1] meist Singular: extreme politische Position, Haltung, Richtung

Examples

Ole

.

Declension

1 Singular Plural
Nominativ der Extremismus die Extremismen
Genitiv des Extremismus der Extremismen
Dativ dem Extremismus den Extremismen
Akkusativ den Extremismus die Extremismen

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary