Der Jasager (literally The Yes Sayer; also translated as The Affirmer or He Said Yes) is an opera (specifically a Schuloper or "school-opera") by Kurt Weill to a German libretto by Bertolt Brecht (after Elisabeth Hauptmann's translation from Arthur Waley's English version of the Japanese Nō drama Taniko).

Its companion piece is Der Neinsager (He Said No) although Brecht's other text was never set by Weill.

Weill also identifies the piece, following Brecht's development of the experimental form, as a Lehrstück, or "teaching-piece".

Brecht subsequently revised the text twice, the final version, including Der Neinsager, being without music.

A teacher leads a hike in the mountains, among the hikers being a young boy whose mother is quite ill. One variant of the story holds that the expedition is a quest for medicine, while another holds that it is in fact a religious pilgrimage.