Cothill Fen is a 43.3-hectare (107-acre) biological and geological Site of Special Scientific Interest north-west of Abingdon-on-Thames in Oxfordshire.

[1][2] It is a Special Area of Conservation[3] and parts of it are a Geological Conservation Review site,[4] a Nature Conservation Review site, Grade 1,[5] and a National Nature Reserve.

[6] It also includes two areas which are nature reserves managed by the Berkshire, Buckinghamshire and Oxfordshire Wildlife Trust, Lashford Lane Fen[7] and Parsonage Moor.

[8] This site has nationally rare calcareous fen and moss-rich mire habitats and a rich invertebrate fauna, including 25 species in the Red Data Book of Invertebrates.

It is a nationally important site geologically because the sampling the peat gives a picture of the vegetation over the early Holocene, between 10,000 and 6,500 years ago.