Why is it der Opportunismus?

Opportunismus 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] Principle behavior in which the principles are abandoned in favor of success

[1] prinzipienloses Verhalten, bei dem Grundsätze zugunsten des Erfolgs aufgegeben werden

Examples

[1] He was threw away limitless opportunism

[1] Man warf ihm grenzenlosen Opportunismus vor.

[1] "What Wedel describes there was not just opportunism"

[1] „Was Wedel da schildert, war nicht einfach Opportunismus.“

Declension

1 Singular Plural
Nominativ der Opportunismus —
Genitiv des Opportunismus —
Dativ dem Opportunismus —
Akkusativ den Opportunismus —

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary