Willow Tree Fen is a local nature reserve with an area of over 112.0 ha (277 acres) located near Bourne and Spalding, south of the River Glen, England.

[1] The site was acquired by Lincolnshire Wildlife Trust in 2009 and converted from an arable field into a wetland fen.

In 2011, an archaeological dig at the site resulted in the discovery of hundreds of historical artefacts, including pottery and tools, from when the reserve was used as a salt making site.

[2] The site was closed to the public in March 2020 as a result of the COVID-19 pandemic in England.

[7] Several bird species that have been noted include bluethroat, spoonbill, red footed falcon, garganey, cuckoo.mammals include brown hare roe deer, otter, water vole .winter is good for winter swans, wigeon and geese, goosanders.