Bear's Den (also called by the name of its most outstanding landmark, Bear's Den Falls) is a 6-acre (24,000 m²) nature reserve in New Salem, Massachusetts. The reservation is close to the Quabbin Reservoir and is owned by the Trustees of Reservations.

The property takes its name from a large black bear once shot here "though more romantic -- yet unsubstantiated -- stories exist" according to the Trustees of Reservations.

The Middle Branch of the Swift River passes through the steep granite cliffs of Bear's Den reservation on its way to the Quabbin Reservoir. The gorge is 75' deep and rimmed with hemlock. At one point, a change in elevation creates a waterfall. According to one description:

So as you walk the 150 yard path from the road and bear left through the jagged opening in the ridge, you have a limited sense of the waterfall being there until you step through and into this amphitheater of Mother Nature. Once inside, you sense an isolation from the rest of the world. It's a great experience and unique for a location so close to the road.