Otter Brook is a 13.2-mile-long (21.2 km)[1] river located in southwestern New Hampshire in the United States.

Otter Brook begins at Chandler Meadow, in the town of Stoddard, New Hampshire.

It flows southwest through the towns of Nelson, Sullivan, and Roxbury, eventually entering the city of Keene, where it joins Minnewawa Brook to form The Branch.

The brook passes through Ellis Reservoir (a small lake in Sullivan) and Otter Brook Lake, a flood control reservoir built on the boundary between Keene and Roxbury.

The dam which created Otter Brook lake was completed in 1958.