Rainham Marshes is an RSPB nature reserve to the east of London, adjacent to the Thames Estuary in Purfleet, Thurrock and the London Borough of Havering. In 2000, the area of land was bought from the Ministry of Defence, who used it as a test firing range. With no activity for several years, the nature reserve was officially opened to the public in 2006. It has maintained much of its Medieval landscape, and is the largest area of wetland on the upper parts of the Thames Estuary.
The site has been designated by Natural England as a Site of Special Scientific Interest as the Inner Thames Marshes, and the part which is in the London Borough of Havering has been designated by the council as a Local Nature Reserve. Wennington, Aveley and Rainham Marshes are a Site of Metropolitan Importance for Nature Conservation.
The reserve is home to a diverse range of bird species, wetland plants and insects. It also has one of the most dense water vole populations in the country. In December 2005, the site was visited by a Sociable Lapwing; over 1,700 people visited the reserve to see this bird. Late in the bird's stay, four Penduline Tits were also found at the site. Entrance to the site is free to residents of Thurrock and the London Borough of Havering.
The site is home to an environmentally friendly visitor centre which features solar panels, rainwater harvesting, natural light and ventilation and a ground heat exchange system. This visitor centre, completed in 2006 at a cost £2 million, was designed by van Heyningen and Haward Architects  The building won six awards for its BREEAM sustainable design, including a Green Apple Award, Regeneration and Renewal Award and a Royal Institute of British Architects National Award.
- List of Sites of Special Scientific Interest in Greater London
- List of Sites of Special Scientific Interest in Essex
- Havering parks and open spaces
- Rainham Marshes photos
- RSBP - Rainham Marshes
-  - Marsh Matters newsletter on the East London Birders Forum