Garrison Tower is a Grade II listed structure on St Mary's, Isles of Scilly The tower was built in the 17th century as a windmill.

By 1750 it was abandoned and in a ruined condition.

The remains were converted to a lookout tower in the 1830s by HM Coastguard.

In 1869, the Shipping and Mercantile Gazette obtained it as a lookout post to report shipping movements.

It was taken over by Lloyd's of London in 1871.