Saltfleetby-Theddlethorpe Dunes is a national nature reserve on the coast of Lincolnshire, England, in the parishes of Saltfleetby and Theddlethorpe.

It is managed in part by Natural England; in part by the Lincolnshire Wildlife Trust, and consists of 1,377 acres (557 ha) of sand dunes, salt marsh, sand and mudflats.

The reserve is one of only around 60 places in the UK where the natterjack toad is found.

Other wildlife includes cuckoo, barn owl and six-spot burnet moth.

A large part of the reserve, close to the car park, is wheelchair-friendly.