Background: The morphology of foot joints is widely accepted as a significant factor in the development of various foot disorders. Nevertheless, the role of the first tarsometatarsal joint (TMT1) morphology in hallux valgus (HV) remains unclear, and its impact on TMT1 instability has not been fully explored. This study aimed to investigate the TMT1 morphology and its potential correlation with HV and TMT1 instability. Methods: Weightbearing computed tomography (WBCT) scans of 82 consecutive feet with HV and 79 controls were reviewed in this case-control study. Three-dimensional (3D) models of TMT1 were constructed using Mimics software and WBCT scans. The height of the TMT1 facet (FH) and the superior, middle, and inferior facet width (SFW, MFW, and IFW) were measured on anteroposterior view of the first metatarsal base. On the lateral view, the inferior lateral facet height and angle (ILFH and ILFA) were measured. TMT1 instability was evaluated using the TMT1 angle. Results: Compared with the control group, the HV group had a significantly wider MFW (9.9 mm in HV, 8.7 mm in control), lower ILFH (1.7 mm in HV, 2.5 mm in control), smaller ILFA (16.3 degrees in HV, 24.5 degrees in control), and larger TMT1 angle (1.9 degrees in HV, 0.9 degrees in control) (all P < .05). No significant differences were found between the 2 groups in FH, SFW, and IFW (all P > .05). The study identified 4 types of TMT1 morphology: continuous-flat, separated-flat, continuous-protruded, and separated-protruded. The continuous-flat type possessed significantly larger HVA, IMA, and TMT1 angles compared with other types (all P < .001). Conclusion: This study indicates a potential association between TMT1 morphology and the severity of HV and identifies 4 TMT1 types. Notably, the continuous-flat type is found to be associated with more severe HV and TMT1 instability.