The halfmoon (Medialuna californiensis), also known as the blue perch, is a species of marine ray-finned fish, a sea chub from the subfamily Scorpidinae, part of the family Kyphosidae.

Most of the head and body is covered in small, thick and rough scales except foe the area forward of the eyes.

[2] The halfmoon is frequently observed high in the water column over rocks and at middle depths over kelp.

The juveniles are occasionally reported far offshore, although they are more frequent around shallow subtidal areas and kelp beds with the adults.

[5] The halfmoon is a prized species for recreational anglers using rod and line, it is a bycatch in traps and bait nets.

[4] The halfmoon was first formally described as Scorpis californiensis in 1876 by the Austrian ichthyologist Franz Steindachner with the type locality given as San Diego, California.