A new genus and species of primitive cryptodiran turtle, Siamochelys peninsularis n. g. n. sp., from the Middle Jurassic Mab Ching locality, in the southern peninsula of Thailand, is described on the basis of complete shells. They are characterized by a combination of primitive features (the presence of a pair of mesoplastra meeting on the midline) and a series of derived characters (loose plastron–carapace attachment, short diamond-shaped entoplastron, dorsal thickening of the lateral edge of the second to seventh peripheral plates, expanded seventh to eleventh peripheral plates, extension of the anal scute to the hyo-hypoplastral suture or onto the hypoplastron, midline sulcus sinusoidal), and thus closely resemble Chengyuchelys, from the Middle Jurassic of China, and more particularly Xinjiangchelys and its relatives from the Middle and Late Jurassic of China and Central Asia. This supports a Middle Jurassic age for the locality and suggests that the Sibumasu (Shan-Thai) block was in contact with the Asian mainland by that time.