Why is it der Redner?

Redner 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] Someone who gives a public speech or speech

[1] jemand, der eine öffentliche Rede oder Ansprache hält

Examples

[1] The speaker made his arguments understandable

[1] Der Redner brachte seine Argumente verständlich vor.

Declension

1 Singular Plural
Nominativ der Redner die Redner
Genitiv des Redners der Redner
Dativ dem Redner den Rednern
Akkusativ den Redner die Redner

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary