It happened near Wells and Kent avenues.
According to the Riverside Police Department, a 44-year-old woman was heading eastbound on Wells Avenue when the horse and its rider walked in front of her 1999 GMC Yukon and were hit by the vehicle.
The impact launched the rider off the horse, according to police. He was rushed to the hospital in critical condition.
The horse was not seriously injured and suffered only minor injuries, according to police.
The driver was not injured and is currently cooperating with investigators.
Police said it's unknown if the driver may have been impaired.
Anyone with information is urged to contact Riverside Police Department Traffic Detective Ryan Mchugh at 951-826-8720.