Doubletop Mountain stands within the watershed of the Delaware River, which drains into Delaware Bay.
The northeast side of Doubletop Mountain drains into the headwaters of Dry Brook, thence into the East Branch of the Delaware River.
The northwest end of Doubletop Mountain drains into Flatiron Brook, thence into Dry Brook.
The southwest side of Doubletop drains into the headwaters of Beaver Kill, thence into the East Branch of the Delaware River.
The south end of Doubletop drains into High Falls Brook, thence into the West Branch of the Neversink River, and the Delaware River.
The southeast slopes of Doubletop drain into Pigeon Brook, thence into Biscuit Brook, and the West Branch of the Neversink.