Tethysaurus is an extinct genus of tethysaurine mosasauroid from the Early Turonian (Late Cretaceous) period.

It was found in the Akrabou Formation, near the villages of Tadirhourst and Asfla in the region of Goulmima, Errachidia Province, in Morocco, with three referred specimens that included a nearly complete articulated skull, mandible, vertebrae and portions of the appendicular skeleton.

The diagnosis after Bardet et al. is "(...) prefrontal strongly vaulted in anterior view; parietal exhibits a triangular table ending posteriorly in two pointed pegs overlying the supraoccipital; jugal with a large and wide ascending ramus; medullar floor of the basioccipital pierced by three foramina; splenial with a large notched dorsomedial process; surangular exposed medially ventral to the coronoid; dental formula: 19-20 maxillary, 15-19 pterygoid and at least 19 dentary teeth; large paracotylar and parazygosphenal foramina on vertebrae.

"[1] A mosasaur measuring under 3 m (9.8 ft) long,[1] Tethysaurus displays a number of basal and derived features that led to an initial classification as an intermediate stage between primitive aigialosaurids of the Cenomanian and derived mosasaurids from the Turonian to the Maastrichtian.

[2] Cladogram based in the analysis by Makádi et al. in 2012:[3] Pannoniasaurus inexpectatus Tethysaurus nopcsai Yaguarasaurus columbianus Russellosaurus coheni