Portchester is a locality and suburb 6 miles (10 km) northwest of Portsmouth, England. It is part of the borough of Fareham in Hampshire. Once a small village, Portchester is now a busy part of the expanding conurbation between Portsmouth and Southampton, on the A27 main thoroughfare. Its population according to the 2001 Census was approximately 17,000 residents.
The fort of Portus Adurni is considered the best preserved Roman fort north of the Alps. The name Portchester is of partly Roman origin, from Latin portus (a harbour) and Old English ceaster (Roman fort). The medieval Portchester Castle was built within the fort.
As well as the castle, its Grade I Listed parish church St Mary's and the abundance of historic houses in Castle Street, this suburb is well placed for waterfront leisure activities, only a short distance from the UK's third largest marina at Port Solent, and from the historic city of Portsmouth and the market town of Fareham.