Windsor Hill Marsh (grid reference ST619454) is a 0.84 hectare biological Site of Special Scientific Interest, north of the town of Shepton Mallet in Somerset, and adjacent to the Windsor Hill Quarry geological Site of Special Scientific Interest.

Windsor Hill Marsh is a marshy silted pond, with adjacent damp, slightly acidic grassland.

The site is of interest for its diverse flora, in large part down to the varied habitats present within the small area.

Two species are present which are rare in Somerset: flatsedge (Blysmus compressus) and slender spike-rush (Eleocharis uniglumis).

Slender spike-rush was discovered in 1972 by Florence Gravestock, and is only found at one other site in Somerset, West Sedgemoor.