Thornaby-on-Tees, commonly referred to as Thornaby, is a town and civil parish on the River Tees's southern bank. It is in the Borough of Stockton-on-Tees, North Yorkshire, England. The parish had a population of 24,741 at the 2011 census, in the Teesside built-up area.

The River Tees, in England, rises on the eastern slope of Cross Fell in the North Pennines and flows eastwards for 85 miles (137 km) to reach the North Sea in the North East of England. The modern day history of the river has been tied with the industries on Teesside in its lower reaches, where it has provided the means of import and export of goods to and from the North East England. The need for water further downstream also meant that reservoirs were built in the extreme upper reaches, such as Cow Green.