Like its parent road, US 9, NY 9D runs north-south parallel to the Hudson River for its entire length. The route begins near the riverbank at the Bear Mountain Bridge in the Westchester County town of Cortlandt, where it meets US 6 and US 202 at the foot of Anthony's Nose. It heads to the northeast as a two-lane road known as the Bear Mountain-Beacon Highway, passing through dense forests in the undeveloped northwestern part of Camp Smith. After just a quarter-mile (0.4 km), NY 9D leaves the military reservation as it passes into Putnam County and the town of Philipstown. Here, the route crosses the Appalachian Trail and runs adjacent to part of Hudson Highlands State Park, a preserve covering three non-contiguous areas between Peekskill and Beacon.