Portrack Marsh Nature Reserve is a 50 acres (20 ha) reserve by the northern bank of the River Tees between the Tees Barrage and the Tees Viaduct, near Portrack housing estate in Stockton-on-Tees borough, County Durham. It is the last remaining wetland on the lower Tees. The site is bounded by Marston Road, a disused railway line, the Northumbrian Water's waste water treatment site, the River Tees, the Tees Barrage White Water Course, the grounds of The Talpore pub and a Tees Barrage access road.

Stockton-on-Tees is a market town in County Durham, England, with a population of 196,000 at the 2021 UK census. It gives its name to and is the largest settlement in the wider Borough of Stockton-on-Tees. It is part of Teesside and the Tees Valley, on the northern bank of the River Tees.