| Salmon Brook | |
| Pushpin Map: | Massachusetts#New Hampshire#USA |
| Pushpin Map Size: | 250 px |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | States |
| Subdivision Name2: | Massachusetts, New Hampshire |
| Subdivision Type3: | Counties |
| Subdivision Name3: | Middlesex, MA, Hillsborough, NH |
| Subdivision Type4: | Communities |
| Subdivision Name4: | Dunstable, MA, Nashua, NH |
| Length: | 9.3miles |
| Discharge1 Avg: | 150ft3/s |
| Source1: | Lower Massapoag Pond |
| Source1 Location: | Groton, MA |
| Source1 Coordinates: | 42.6686°N -71.4942°W |
| Source1 Elevation: | 154feet |
| Mouth: | Merrimack River |
| Mouth Location: | Nashua, NH |
| Mouth Coordinates: | 42.7486°N -71.4417°W |
| Mouth Elevation: | 95feet |
| Basin Size: | 36sqmi |
| Tributaries Left: | Hauk Brook, Joint Grass Brook, Hassells Brook |
| Tributaries Right: | Black Brook, Old Maids Brook |
Salmon Brook is one of the six major tributaries of the Merrimack River in northeastern Massachusetts in the United States. Its watershed is 31sqmi and is one of the 14 subwatersheds of the Merrimack River. It passes through Dunstable, Massachusetts, and Nashua, New Hampshire.
Salmon Brook begins at the outlet of Lower Massapoag Pond near the center of Dunstable. It then flows approximately 9.3miles north-northeast to the Merrimack River. The water body farthest upstream in the watershed is Martins Pond just east of the center of the town of Groton. The outlet, Martins Pond Brook, runs southeast to Lost Lake/Knops Pond, the start of a chain of lakes that includes Whitney Pond, Cow Pond, and Upper and Lower Massapoag ponds. All of these water bodies are dammed. Salmon Brook runs roughly parallel to the Nashua River for its entire course from Lower Massapoag Pond to the Merrimack River.