Quercus cornelius-mulleri is a North American species of oak known by the common name Muller oak, or Muller's oak. It was described to science in 1981 when it was segregated from the Quercus dumosa complex and found to warrant species status of its own. It was named for the ecologist Cornelius Herman Muller. It is native to southern California and Baja California, where it grows in chaparral, oak woodlands, and other habitat in foothills and mountains. It can most easily be observed in Joshua Tree National Park and in the woodlands along the western margins of the Colorado Desert in San Diego County, California.