Lyme Bay is an area of the English Channel situated in the southwest of England between Torbay in the west and Portland in the east. The counties of Devon and Dorset front onto the bay.

The area around Lyme Bay is part of a World Heritage Site, the Jurassic Coast, named for its Jurassic geology. The Jurassic Coast runs from near Swanage in the Isle of Purbeck to near Exmouth in East Devon. Many of the earliest discoveries of dinosaur and other prehistoric reptile remains were in the area surrounding Lyme Regis and Charmouth. Notable among these were the discoveries made by self-educated paleontologist and fossil collector Mary Anning, in the 1820s.

The weather in the bay is temperate by English standards, and far more temperate than many other places at a similar latitude. The reason for this is the warming action of the Gulf Stream. The area along the coast of Lyme Bay is thus a popular holiday destination. On 22 March 1993, four schoolchildren died in what is known as the Lyme Bay kayaking tragedy.

Lyme Bay was the site of Exercise Tiger, a practice run for the D-Day invasion of France in 1944, using the beach called Slapton Sands near Slapton, Devon as the practice landing area. The operation went horribly wrong when German E-boats appeared on the scene and killed 749 American Army and Navy personnel in the middle of the bay.

A new ship currently under construction for the Royal Fleet Auxiliary carries the name of the area, RFA Lyme Bay.

The beached container ship, MSC Napoli, is visible from Lyme Bay.