Picrocleidus is an extinct genus of plesiosaur from the Middle Jurassic Oxford Clay Formation (Callovian stage) of the United Kingdom.

The holotype was discovered in the Oxford Clay Formation in Peterborough, England and acquired by Alfred Nicholson Leeds.

[2][4] The generic name is derived from Greek pikros, "sharp", en kleis, "key", again referring to the clavicles.

[2] Specimen BMNH R2739, consisting of a basisphenoid and parasphenoid, cervical vertebrae, sacrals with the sacral ribs, an anterior caudal, some ventral ribs, both radii, odd paddle-bones, portions of ilia, ischia, and pubes.

Picrocleidus has previously been considered synonymous with Muraenosaurus although there are doubts in the relationship between the two groupings.