Witnesses say the horse bucked the rider off the saddle, hitting the asphalt.
Sheriff Vern Warn-key told Action News the rider fell off the horse and suffered a "significant injury."
No one else was hurt in the fall.
The Merced County Sheriff's Posse is comprised of volunteers who serve as ambassadors for the Sheriff's Office, Merced County and the State.
The Posse travels over 3,000 miles around the community each riding on Palomino horses.