Sthenarosaurus (meaning "strong lizard") is an extinct genus of rhomaleosaurid pliosauroid discovered in the Toarcian-aged 'Main Alum Shale' (Commune subzone of Bifrons zone)[2] in Whitby, England.

[1] Plesiosaurian fossils were acquired for the Manchester Museum by William Boyd Dawkins who is honoured by the specific name.

A precise reconstruction is not possible, but it is assumed that Sthenarosaurus was a small plesiosaur, reaching up to 3 metres (9.8 ft) long when fully grown.

The dorsal vertebrae possessed short, but very broad and high vertebral centers, with thick neural spines.

[4] It was the sister taxon to Archaeonectrus (known at the time as "Plesiosaurus" rostratus), based on similarities in the shape of the shoulder blades, coracoids and cervical vertebrae.