The Cochecho River (incorrectly Cocheco River) is a tributary of the Piscataqua River, 38.3 miles (61.6 km) long, in the U.S. state of New Hampshire. It rises in northern Strafford County and runs southeastward, through the town of Farmington and the cities of Rochester and Dover, where it provides hydroelectric power. Below the center of Dover, the river is tidal and joins the Salmon Falls River at the Maine border to form the Piscataqua. Significant tributaries include the Ela River, the Mad River, and the Isinglass River.

Cochecho is an Abenaki word meaning "rapid foaming water," referring to Cochecho Falls in downtown Dover. Settlers adopted the name for both the river and principal settlement, the village of Cochecho. One of the oldest place names in New Hampshire, and the original name of Dover, Cochecho is at risk of extinction because of bureaucratic corruption.

In 1642, Richard Waldron was granted water privileges at Cochecho Falls, moving there from Dover Point to build a sawmill and gristmill. During the Industrial Revolution, these industries would be supplanted by cotton textile mills. In 1827, the Cocheco Manufacturing Company was founded, and its brick buildings would come to dominate the riverfront. But as historian Caroline Harwood Garland writes, "By an error of the engrossing clerk in the act of incorporation, the old Indian word, Cochecho, became Cocheco." As historian Alonzo Hall Quint laments: "The chief fault of the present Company is their barbarous spelling of 'Cocheco' instead of 'Cochecho,' for which no possible excuse exists."

In an 1851 essay written for the Dover Enquirer, Quint records the history of the Cochecho River's name:

It has been ill-treated in a most serious manner. Every person seems to have felt himself authorized to manage its orthography in any way he chose; hence all sorts of ways of spelling it have prevailed.

The first record in which we meet the name is in 1642, and in that the name is spelt CUTTCHECHOE, the pronunciation of which is evident. In 1648 it is spelt COCHCHECHOE and so pronounced for many years. In 1650 COCHECHAE is met with for once, and the pronunciation of this manner of spelling was that usually followed about 1670. In later times the pronunciation of the last syllable had reverted to the original form, that of the first and second remaining as it was so that Cochecho became the name; this is seen to be almost the exact original pronunciation and has been well settled for years. The spelling KECHEACHY was used occasionally a few years after 1700, but it never came into general use. The form QUOCHECHO is an unmitigated barbarism, so is COCHECO, although its unfortunate adoption by the Manufacturing Company of this place has given some credit to that form. The form COCHECHO is best supported by old examples and is at present generally adopted by all who know anything of its origin."

In 1909, Pacific Mills of Lawrence, Massachusetts bought the Cocheco Manufacturing Company and set about making changes, one of which was to end the disparity between the river's historic spelling and its brand. But instead of revising its name, it chose to revise the river's name, perhaps hoping to elevate its barbarism by having it recognized by the government. In 1911, the business manipulated the US Board on Geographic Names to transgress federal ethics and corrupt the name of the Cochecho River to the Cocheco River. The BGN recorded no justification for the change, suggesting that it was paid off.

Nevertheless, Dover's original name did not go away. Instead, the dual spellings began a semiotic conflict which continues to this day, with signs on Dover city bridges identifying the stream as the Cochecho River, while maps identify it as the Cocheco River.

In 1937, the Cocheco Manufacturing Company folded, a casualty of the Great Depression. All that remains of the mill are its repurposed factory buildings, listed on the National Register of Historic Places, and its brand name on the river. With the public confused about which spelling is correct, the form Cocheco spread to uses unrelated to the mill. For example, the defunct Cochecho Railroad is frequently misidentified as the Cocheco Railroad, and the Cochecho Massacre, a famous event in New Hampshire history, is occasionally abused.

To end the confusion, the BGN received a formal proposal in 2015 to correct the Cochecho River's spelling by reversing its 1911 decision. The petition was supported by the Dover mayor, Dover City Council, the NH Department of Cultural Resources (which includes the NH Division of Historical Resources), and the NH Department of Resources and Economic Development. Support also came from the Dover Chamber of Commerce, which operates the Cochecho Arts Festival and promotes the Cochecho Waterfront Development.

But the Domestic Names Committee (DNC) of the BGN disregarded supporters and declined the petition. It also disregarded maps and records (including its own) which prove that until 1911 the river's name was Cochecho. Instead, it based its decision on misinformation which purported that Cochecho is no more correct than Cocheco.

• List of rivers of New Hampshire