The fossil record of the ichthyosaur genus Stenopterygius comprises a large number of specimens of all ontogenetic stages. This makes the taxon a good model for ontogenetic studies in a Mesozoic marine reptile. Here, we describe the morphology of the braincase elements of Stenopterygius over ontogeny and compare it with that of other known ichthyosaur braincases. Substantial ontogenetic changes were observed in most elements. These included negative allometry of the exoccipitals relative to the basioccipital, changes in the morphology of the parabasisphenoid and proportional reduction of the ridge separating the right and left internal carotid arteries, variable relative length of the paroccipital process of the opisthotic in juvenile specimens, and relative ossification of the basioccipital peg. Furthermore, we observed variability in relative braincase ossification between embryos within a single gravid female.