San Sevaine Flats is a small area of flatland east of Cucamonga Peak in the San Gabriel Mountains in San Bernardino County, California. The area is in the Cucamonga Wilderness in the San Bernardino National Forest, 1.24 miles south of Bonita Falls on South Fork Lytle Creek and north of Rancho Cucamonga, California. It has an elevation of 1,690 meters, or 5,545 feet.