The Cosmos Mercury was a fourteen-cylinder twin-row air-cooled radial aeroengine.

Designed by Roy Fedden of Cosmos Engineering, it was built in the United Kingdom in 1917.

It did not enter production; a large order was cancelled due to the Armistice.

It was said to run well without vibration and set an unofficial time to climb record while fitted to a Bristol 21A Scout F.1, the aircraft achieving 10,000 ft (3,000 m) in 5.4 minutes and 20,000 ft (6,000 m) in 16.25 minutes.

[2] The name was re-used by Fedden for the later nine-cylinder Bristol Mercury radial engine.