Astronotus crassipinnis is a South American fish in the cichlid family from the southern Amazon basin and the Paraná–Paraguay basins. It is not as well-known or common in the aquarium trade as its relative, the more northernly distributed oscar (A. ocellatus). A. crassipinnis reaches up to 25 cm in length.

Etymology: The genus name Astronotus comes from the Greek words astra ("ray") and noton ("back"). The species name crassipinnis comes from the Latin words crassus ("fat") and pinna ("fish").

In the aquarium: The species is very rare in the aquarium hobby, where they are sometimes known as "fat oscars". When they appear, they are often mistaken for the common oscar (A. ocellatus).