Nashua River, just outside Groton, Massachusetts
|- location||Lancaster, Massachusetts|
|- elevation||280 feet (85 m) at Clinton, Massachusetts|
|Length||37.5 miles (60 km)|
|Basin size||108 square miles (280 km2)|
|- average||150 cu ft/s (4.2 m3/s)|
NASHUA RIVER Latitude and Longitude:
The Nashua River, 37.5 miles (60.4 km) long,  is a tributary of the Merrimack River in Massachusetts and New Hampshire in the United States. It is formed in eastern Worcester County, Massachusetts, by junction of its north and south branches near Lancaster, and flows generally north-northeast past Groton to join the Merrimack at Nashua, New Hampshire. The Nashua River Watershed occupies a major portion of north-central Massachusetts and a much smaller portion of southern New Hampshire.
The north branch rises west of Fitchburg and Westminster. It flows about 30 miles (48 km) generally southeast past Fitchburg, and joins the south branch about 5 miles (8.0 km) below its issuance from the Wachusett Reservoir.
The Nashua River was heavily used for industry during the colonial period and the early United States. During the late 18th century and early 19th century, the heavy concentration of paper mills and the use of dyes near Fitchburg resulted in pollution that notoriously turned the river various colors downstream from the factories.
In the mid-1960s, Marion Stoddart started a campaign to restore the Nashua River and its tributaries. She built coalitions with labor leaders and business leaders, in particular the paper companies who were the worst polluters of the river. With federal help, eight treatment plants were built or upgraded along the river. A broad conservation buffer was created along about half the river and its two main tributaries. By the early-1990s, most of the industry was still located along the river, but many parts of the river were once again safe for swimming. Her work is the subject of a 30-minute documentary movie. 
Recovery has sparked recreational use of the river at places like Mine Falls Park in Nashua.
On January 23, 2013, Rep. Niki Tsongas introduced the Nashua River Wild and Scenic River Study Act (H.R. 412; 113th Congress), a bill that would amend the Wild and Scenic Rivers Act to designate certain segments concerning the Nashua River for study for potential addition to the National Wild and Scenic Rivers System. 
From its impoundment at the Wachusett Reservoir in Clinton, Massachusetts, the South Branch of the Nashua River flows north and joins the North Branch of the Nashua River in Lancaster, Massachusetts. The North Branch of the Nashua River flows southeast from Fitchburg and Leominster, Massachusetts, to Lancaster. The Nashua River flows northward from Lancaster, meandering its way through the north-central Massachusetts towns of Harvard, Groton, Dunstable, and Pepperell, before eventually emptying into the Merrimack River at Nashua, New Hampshire. The Nashua River Watershed has a total drainage area of approximately 538 square miles (1,390 km2), with 454 square miles (1,180 km2) of the watershed occurring in Massachusetts and 74 square miles (190 km2) in New Hampshire. The Nashua River flows for approximately 56 miles (90 km), with approximately 46 of those miles (74 km) flowing through Massachusetts. The Squannacook, Nissitissit, Stillwater, Quinapoxet, North Nashua, and South Nashua rivers feed it. The watershed encompasses all or part of 31 communities, 7 in southern New Hampshire and 24 in central Massachusetts. The watershed’s largest water body is the Wachusett Reservoir, which provides drinking water to two-thirds of the Commonwealth's population. 
- Wachusett Reservoir is the largest water body.
- Wachusett Reservoir Dam impounds the Nashua River.
- The North Nashua River joins the main river at Lancaster from the northwest.
- The Squannacook River joins the Nashua River from the northwest at the junction of the town boundaries of Shirley, Groton, and Ayer.
- The Nissitissit River joins the Nashua River from the northwest at Pepperell.
|River system||Drainage area||Communities|
|Stillwater River||39.3 square miles (102 km2)||Princeton, Leominster, Sterling, and West Boylston, Massachusetts|
|Quinapoxet River||57 square miles (150 km2)||Princeton, Rutland, Paxton, Holden, and West Boylston, Massachusetts|
|North Nashua River||Gardner, Ashburnham, Westminster, Ashby, Fitchburg, Lunenburg, Leominster, Sterling, and Lancaster, Massachusetts|
|Squannacook River||73 square miles (190 km2)||Greenville, New Ipswich, and Mason, New Hampshire, plus West Groton, Shirley, Townsend, and Ashby, Massachusetts|
|Nissitissit River||23 square miles (60 km2)||Wilton, Mason, Milford, Brookline and Hollis, New Hampshire, plus Pepperell, Massachusetts|
- List of rivers of Massachusetts
- List of rivers of New Hampshire
- Nashua River Rail Trail
- Nashaway people
- U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National Map, accessed October 3, 2011
- "Work of 1000" movie
- Nashua River Watershed Association
- "H.R. 412 - Summary". United States Congress. Retrieved 23 June 2014.
- "Nashua River Watershed". Retrieved 2007-01-03.
- "The Promise of Restoration", National Geographic Magazine, November, 1993, Special Edition: Water.
- Parfit, Michael, "New Ideas, New Understanding, New Hope", National Geographic Magazine, November, 1993, Special Edition: Water.