San Mateo Peak is an unofficially named 3,591-foot mountain in the Elsinore Mountains, within California’s Santa Ana Mountains. At the western end of a ridge extending from Elsinore Peak toward Morrell Canyon, it is the highest point in the Elsinore Mountains, rising above the subrange’s highest named summit, Elsinore Peak. The name was used in the Sierra Club Lower Peaks Committee guide and originated with Ken Croker, who maintained Santa Ana Mountain trails for 20 years with Sierra Club volunteers. The peak also forms a local drainage divide, with waters flowing toward San Mateo Creek, Morrell Canyon Creek, and nearby arroyos. Its importance rests in being both the range’s tallest summit and officially nameless.