Narziß Kaspar Ach (29 October 1871 – 25 July 1946) was a German psychologist and university lecturer in Königsberg, Prussia and Göttingen, Germany.

Ach was a member of the Würzburg School of "imageless thought", where he was a student of Oswald Külpe and received his PhD in 1899.

Considered one of the most skillful experimental psychologists of his time, Ach was the first to conduct experiments in the area of volition.

He developed the method of "systematic experimental introspection" and demonstrated experimentally the "determining tendency," which indicates that the instructions for the execution of a task sets up in a person a predisposition to act or react in a certain way, without the individual's being necessarily aware of it.

[1] In 1933 Ach signed the Loyalty Oath of German Professors to Adolf Hitler and the National Socialist State.