Authorities released a video of the attack that occurred just before 11 a.m. on May 10 in the 200 block of South Central Avenue.
Police say the suspect walked up to the victim and attacked him.
The victim, a homeless man, suffered serious injuries after being punched repeatedly in the face.
The suspect was described by police as either a white or Hispanic male, 30 to 40 years of age, medium build and clean shaven. He was last seen wearing a black cap, a dark blue tank with white stripes, and khaki-colored pants.
Anyone with information is urged to call the Glendale Police Department at (818) 548-4911. Those wishing to remain anonymous can call Crime Stoppers at (800) 222-8477.