Southill Lake and Woods is a 25.3-hectare (63-acre) biological Site of Special Scientific Interest in Southill in Bedfordshire.

[1][2] It is part of Southill Park, which was designed by Capability Brown, and is registered by English Heritage for its special historic interest, and is the garden of a house of the same name.

[3] The wood is a wet valley of alder, fed by springs, and a small stream runs down to the lake.

There is fen vegetation in more open areas.

The lake has a characteristic population of breeding birds, and an island has one of only two surviving heronries in the county.