A new specimen of Stenorhynchosaurus munoziPáramo-Fonseca et al., 2016 (Plesiosauria, Pliosauridae), found at Sáchica (Boyacá, Colombia) in upper Barremian beds of the Arcillolitas Abigarradas Member of the Paja Formation, is described herein. It consists of a complete skull and 10 cervical vertebrae of an adult individual and provides valuable new morphological information on the species: the presence of nasals covering a subsurface circular cavity where salt glands were probably lodged (new autapomorphy), the presence of a high sagittal crest on the parietal, a very elongated pineal foramen enclosed entirely within the parietals, and unfused neural arches in the cervical vertebrae. A phylogenetic analysis supports the inclusion of Stenorhynchosaurus munozi within the pliosaurid clade Brachaucheninae. A morphological comparison between the juvenile and adult stages of S. munozi shows that in the adult stage, the margins of the internal nares and the anterior part of the anterior interpterygoid vacuity are well ossified, the medial joints of the pterygoids and the mandibular symphysis are closed, and the atlas and the axis are fused, but the cervical neural arches remain unfused to the centra. The growth pattern of the skull dorsal bones differs from that of the palatal elements, but the relative distance between the external and internal nares remains the same. A greater longitudinal growth in the skull and a negative allometric orbital growth pattern results in a proportionally longer skull and relatively smaller orbits in the adult.