Microcleidus is an extinct genus of sauropterygian reptile belonging to the Plesiosauroidea. The species has 40 neck vertebrae and a short tail of 28 vertebrae. Fossils of the genus have been found in France, the Posidonia Shale in Germany and Luxembourg, and the Alum Shale Formation of England.

The type species, M. homalospondylus, was the largest, measuring 5.1 m (17 ft) long and weighing 650 kg (1,430 lb). Other species were smaller: M. tournemirensis was about 4 m (13 ft) long and weighed 300 kg (660 lb), and M. melusinae was about 3 m (9.8 ft) long and weighed 120 kg (260 lb).

Species include: Microcleidus homalospondylus (Owen 1865) and Microcleidus macropterus (Seeley 1865).

Occitanosaurus tournemirensis (originally "Plesiosaurus" tournemirensis), was named by Sciau et al. in 1990, based on a nearly complete skeleton of an animal approximately 4 meters (13 ft) long. It was later found to be a species of Microcleidus.

The following cladogram follows an analysis by Ketchum & Benson, 2011.