Bourn Windmill is an open trestle post mill at Bourn in Cambridgeshire, England, which was standing in 1636. It is a Grade I Listed building and a Scheduled Monument. It is the oldest surviving windmill in the United Kingdom. The mill ceased work commercially in 1927 and was preserved in 1932. In November 2021, it was placed on the Heritage at Risk Register as being in danger of collapse.