Cornish Flat is an unincorporated community in the town of Cornish in Sullivan County, New Hampshire, United States. The village is located in the northeastern corner of Cornish, at the southern end of a valley floor which is bordered westerly by Cornish Stage Road, easterly by New Hampshire Route 120, and whose northerly end is in the town of Plainfield. The headwaters of Blow-me-down Brook gather in a swampy lowland at the valley's north end formed by the confluence of Notch, Leavitt, Wine, and Penniman brooks entering the valley from the uplands of Corbin Park (a private game preserve) to the east and southeast, and by seasonal streams from the west side of the valley. Route 120 connects the village with Claremont to the south and Meriden and Lebanon to the north.