Stodmarsh SSSI is a 623.2-hectare (1,540-acre) biological Site of Special Scientific Interest near Stodmarsh, north-east of Canterbury in Kent. Parts of it are a Nature Conservation Review site, Grade I, a National Nature Reserve, a Ramsar internationally important wetland site, a Special Area of Conservation and a Special Protection Area under the European Union Directive on the Conservation of Wild Birds.

Stodmarsh NNR has an area of 250 hectares (620 acres) and lies within the SSSI. The reserve is open to the public. It consists of a number of different habitats that are important for wildlife:

It is designated as one of only 35 "spotlight reserves" in England by Natural England in the list of national nature reserves in England. This is not strictly speaking a "natural" habitat – the area around Grove Ferry has been restored to wetland habitat by English Nature (now Natural England) and the areas of open water came about as a result of the flooding of areas used for gravel extraction or undermined by mining subsidence.

The Stodmarsh NNR now forms a central and ecologically important feature in the Kentish Stour Countryside Project and is an important site for the protection and encouragement of critically endangered aquatic mammal species such as the European otter and the water vole.

The Nature Reserve car park, operated by Natural England is along a lane from Stodmarsh village (grid ref TR221609, postcode CT3 4BE). There is a small voluntary charge for parking (as of 2022). Alternative parking is available at Grove Ferry (TR236631, CT3 4BP), operated by Kent County Council, where there is an obligatory charge. A public footpath leads through the reserve between these two points, with bird hides at intervals. Another, longer, path runs along the river to meet the reserve path at each end. The reserve may be visited at any time; entry is free.

The valley of the River Stour around Stodmarsh was for centuries a series of flood meadows used primarily as pasture for cattle and horses. At the beginning of the 20th century, Chislet Colliery was established by the Anglo-Westphalian Coal Syndicate, from where a layer of coal-bearing rocks beneath the valley was mined, which eventually caused subsidence of the surface above. By the 1930s, extensive shallow lakes had formed between Fordwich and the confluence of the Stour and the Lampen stream, causing a cessation of farming in this area. The coal company used some of the land on the south side of the river for dumping spoil from the mine. The presence of large water bodies and stands of reedbed, combined with the relaxation of grazing, created a haven for migratory birds and other wildlife.

Interest in the natural history of the Stodmarsh valley was stimulated in 1947 by the field club of The King's School, Canterbury, which spent a year studying the site under the guidance of arts teacher David Stainer and biology master Cyril Ward. Their report includes a botanical survey by Francis Rose, which describes some of the characteristic species of the area. The SSSI was notified a few years later, in 1951, and the National Nature Reserve was later established on the (former) mining company's land west of the Lampen Wall. Over the next two decades, the NNR was expanded to encompass most of the land eastward along the valley as far as Grove Ferry.

The ecological importance of the reserve was outlined in the Nature Conservation Review in 1977, where it was described as the largest reedbed in southern England and ranked as a Grade 1 site of national importance, primarily for its ornithological interest; most notably for the presence of a small population of bitterns, which thereby became a flagship species for the NNR. In order to enhance the habitat for these birds, hundreds of hectares of reedbed was subsequently created on the flood meadows immediately east of the Lampen Wall in the 1980s and west of the Grove Ferry road in the 1990s.