The town is apparently named for the shape of the lower elevation land north of the junction of the Hudson and Mohawk Rivers.

The southern town line, delineated by the Mohawk River, is the border of Albany County.

The eastern town line, at the Hudson River, is the border of Rensselaer County.

The northern town line is marked by the Anthony Kill, a tributary of the Hudson River.

Interstate 87, the Adirondack Northway, is a six-lane north–south freeway along the western edge of Halfmoon.

U.S. Route 9 parallels the interstate farther to the east and crosses into the town from Albany County on the Crescent Bridge.

New York State Route 146, an east–west highway, intersects US-9 at the hamlet of Clifton Park.