Coston Fen, Runhall is a 7.1-hectare (18-acre) biological Site of Special Scientific Interest between Dereham and Wymondham in Norfolk. It is part of the Norfolk Valley Fens Special Area of Conservation.

This spring-fed site in the Yare valley has a variety of fen habitats, including a nationally rare calcareous mire community of fen flora. There are also areas of tall herbs, scrub and improved pasture.

The site is private land with no public access.