Great Gaddesden /ˈɡædzdən/ is a village and civil parish in Dacorum Hundred in Hertfordshire, England.

The parish borders Flamstead, Hemel Hempstead, Nettleden and Little Gaddesden and also Studham in Bedfordshire.

Part of the chancel with Roman bricks dates back to the early 12th century.

For a while the site served as a WW2 Royal Canadian Air Force transit camp and later a boarding school for children with special needs, and it is now a Theravadin Buddhist monastery of Thai Forest Tradition, the Amaravati Buddhist Monastery, complete with temple.

The bridge over the river at Water End has a medieval appearance but was built in the 19th century.