Martinectes is an extinct genus of polycotylid plesiosaur from the Late Cretaceous Sharon Springs Formation of the United States.

Two very large specimens of a polycotylid plesiosaur (KUVP 40001 and 40002[2]) were collected from the Pierre Shale of Wyoming and later reported on by Adams in her 1997 Masters thesis, and in the same year, she officially described the specimens as a new species of Trinacromerum (T. bonneri).

[6] A 2023 study assigns D. bonneri to a new genus, Martinectes; the name means "Martin's swimmer".

[1] Clark, O'Keefe & Slack (2023) recovered Martinectes as a polycotylid member of the plesiosaur clade Leptocleidia, as the sister taxon to an unnamed polycotyline from the Niobrara Formation.

The results of their phylogenetic analyses are shown in the cladogram below:[1] Scalamagnus Dolichorhynchops herschelensis Martinectes ROM 29010 (Niobrara polycotyline)[7] Unktaheela