Thornaby-on-Tees, commonly referred to as Thornaby, is a market town and civil parish in Stockton-on-Tees, North Yorkshire, northern England. It had a population of 24,741 according to the 2011 census, in the Teesside built-up area. It lies south of the River Tees.
The town had a royal charter enacted to form a municipal borough before being a part of the Borough of Stockton-on-Tees.
The modern centre was built on the north eastern part of Thornaby airfield and lies 2 miles (3.2 km) south-west of Stockton-on-Tees and 4 miles (6.4 km) south-west of Middlesbrough.