BACKGROUND CONTEXTSex diversity in the spine surgery workforce remains limited. Accelerated efforts to recruit more female trainees into spine surgery fellowship training may help promote diversity and inclusion in the emerging spine surgery workforce. PURPOSEThis study assessed the representation of female trainees in spine surgery fellowship training and program factors associated with greater sex diversity among fellows. Study Design/SettingThis was a cross-sectional analysis of spine surgery fellows in the United States during the 2016–2017 to 2022–2023 academic years. PATIENT SAMPLEN/A. OUTCOME MEASURESRepresentation (%) and participation-to-prevalence ratios (PPRs) defined as the participation of female trainees in spine surgery fellowship training divided by the prevalence of female trainees in previous training cohorts. PPR values <0.8 indicated underrepresentation. METHODSSex diversity was assessed among spine surgery faculty, spine surgery fellows, orthopaedic surgery residents, neurosurgery residents, and allopathic medical students. Fellowship program characteristics associated with increased sex diversity were calculated with chi square tests. RESULTSThere were 693 spine surgery fellows and 41 were female (5.9%). Sex diversity in spine surgery fellowship training decreased over the study period (6.4% vs. 4.1%, p=.025). Female trainee representation in spine surgery fellowship training was less than that in orthopaedic surgery residency (14.2%, PPR=0.42), neurosurgery residency (17.1%, PPR=0.35), and allopathic medical school (47.6%, PPR=0.12) training (p<.001). There were 508 faculty at 78 spine surgery fellowships and 25 were female (4.9%). There were 3 female fellowship program directors (3.8%). Fellowship program characteristics associated with increased sex diversity included the presence of female faculty (p=.020). Additional program characteristics including geographic region, accreditation status, number of faculty and fellows were not associated with sex diversity (p>.05). CONCLUSIONSFemale representation in spine surgery fellowship training decreased over the study period and remains underrepresented relative to earlier stages of medical and surgical training. There was a positive association between female faculty and increased sex diversity among fellows. Greater efforts are needed to create training environments that promote diversity, equity, and inclusion in spine surgery fellowship training.