Why is it der Solist?

Solist ends in -ist, and nouns ending in -ist are masculine: in a German dictionary, 97% of them take der.

All nouns ending in -ist
Sense 1
der

Meaning

[1] Art: Someone who sings, dances or plays a solo

[1] Kunst: jemand, der ein Solo singt, tanzt oder spielt

Examples

[1] The soloists did their cause very well today

[1] Die Solisten haben ihre Sache heute sehr gut gemacht.

Declension

1 Singular Plural
Nominativ der Solist die Solisten
Genitiv des Solisten der Solisten
Dativ dem Solisten den Solisten
Akkusativ den Solisten die Solisten

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary