In this study, we analysed the ontogenetic trajectories of septal spacing between succeeding chambers of the gaudryceratid ammonoid, Gaudryceras tenuiliratum, which were excavated in the Tomamae and Mikasa areas of Hokkaido, Japan. The ontogenetic trajectories of septal spacing in G. tenuiliratum demonstrate a general trend: two cycles of increasing to decreasing septal spacing until the ~30th septum, gradually decreasing septal spacing until the ~70th septum, and then gradually increasing septal spacing at least until the ~110th septum. The ontogenetic trajectories of the whorl expansion rate (WER) in our specimens also demonstrated that a decreasing trend changed into an increasing trend at a shell diameter of ~5–7 mm. This shell diameter corresponds to the end of the second cycle of increasing to decreasing septal spacing occurring before the ~30th septum, at which point G. tenuiliratum possibly transitioned from planktic to nektobenthic habits. No significant differences were detected in the ontogenetic trajectories of septal spacing and conch shape between the two areas, which suggests that the ontogenetic trajectory patterns of septal spacing in the Late Cretaceous ammonoids were taxonomy-dependent rather than environment-dependent.