Blyth is a port and seaside town as well as a civil parish in southeast Northumberland, England. It lies on the coast, to the south of the River Blyth. It has a population of 39,731 as of the 2021 census, up 6% from the 2011 census and population of 37,347. The port of Blyth dates from the 12th century, but the development of the modern town only began in the first quarter of the 18th century.