Cliff Mountain is a mountain located in Essex County, New York. The mountain is part of the Marcy Group of the Great Range of the Adirondack Mountains. Cliff is flanked to the southeast by Mount Redfield. Cliff Mountain stands within the watershed of the Opalescent River, a tributary of the Hudson River, which in turn drains into New York Bay. The north and west sides of Cliff Mtn. drain directly into the Opalescent River. The southeast side drains into Upper Twin Brook, thence into the Opalescent River. According to the 1897 survey of the Adirondacks, the height of Cliff Mountain was over 4000 ft, so it was included in the 46 High Peaks; the 1953 USGS survey found it and three other peaks to be lower, but the list has not been changed. The mountain is within the High Peaks Wilderness Area of Adirondack State Park. No marked trail leads to its summit.