An excellently preserved partial skeleton of a rhomaleosaurid plesiosaurian (NLMH 106.058) from the Sinemurian (Lower Jurassic) of Lyme Regis, England, is described. The material consists of a complete cranium, mandible, and articulated cervical vertebral column, and it is noteworthy because large-headed rhomaleosaurids are rare from this stratigraphic horizon and it is taxonomically distinct. The material is referred to a new taxon, Thaumatodracon wiedenrothi gen. nov. et sp. nov, diagnosed by the two autapomorphies: 1. a pronounced transverse trough on the posterior margin of the dorsal ramus of the squamosal; 2. possibly paired anteriorly tapering triangular basioccipital processes. It also possesses a unique combination of other characters including a 'short' premaxillary rostrum (length and width subequal), five premaxillary alveoli, premaxilla-maxilla sutures parallel anterior to the external nares, frontals contact on the midline, prefrontal-frontal suture convex and gently curved medially, mandibular symphyseal region spatulate and 'short' (length and width subequal), prominent dorsally concave medial flange anteromedial to the articular glenoid, robust rod-like axis neural spine with a circular transverse cross section, and cervical neural spines with a mediolaterally expanded apex. The taxon shares some of these characters with earlier Hettangian rhomaleosaurids (e.g. Atychodracon, Eurycleidus), and other characters with later Toarcian rhomaleosaurids (e.g. Rhomaleosaurus sensu stricto and Meyerasaurus). Inclusion of Thaumatodracon as an additional operational taxonomic unit in several existing cladistic analyses demonstrated that it occupies a relatively derived position within Rhomaleosauridae. A morphometric multivariate analysis of Lower Jurassic rhomaleosaurids shows that Thaumatodracon is also proportionally intermediate between known rhomaleosaurid taxa. Thaumatodracon is therefore a stratigraphically and anatomically intermediate taxon that fills a gap in our knowledge of the evolution of this macro-predatory plesiosaurian clade.