Marble Mountain is a 771-foot mountain in Warren County, New Jersey, within the New York–New Jersey Highlands of the Appalachian Mountains. Its summit lies in Harmony and Lopatcong Townships above the Delaware River. The mountain is significant for standing at the narrow river passage called Weygadt Gap, or the Little Water Gap, which it helps form together with Chestnut Hill on the Pennsylvania side. Though part of the Highlands, Marble Mountain stands somewhat apart, isolated to the west of the range’s main body.