INTRODUCTION: Little is known about the impact of gender-affirming hormone therapy (GAHT) on transgender and gender diverse adults with inflammatory bowel disease (IBD). The primary aim was to evaluate the incidence of IBD flare in the year before and after GAHT initiation. METHODS: A retrospective study across 5 IBD centers. Flare was defined as need for steroids, IBD-associated emergency department visit, or need for IBD medication change. Factors associated with IBD flare were assessed with univariate analysis and multivariable logistic regression controlling for age and IBD type. RESULTS: A total of 85 transgender and gender diverse adults with IBD who initiated GAHT were included in this study. Forty-six (54.1%) received estrogen and 39 (45.9%) received testosterone. In the year before GAHT, 42 (49%) flared compared with 32 (38%) in the year after, P = 0.06. There was no statistically significant difference in incidence of flare by age, IBD type, or IBD therapy type. Individuals with active IBD at GAHT initiation were more likely to flare in univariate (58% vs 24%, P = 0.003) and multivariable analyses (adjusted odds ratio 5.1, 95% confidence interval 1.7-15.2). In both univariate and multivariable analyses, individuals who received testosterone were more likely to flare in the year after starting GAHT, testosterone: 51% vs estrogen: 26%, P = 0.02 with an adjusted odds ratio 3.1 (95% confidence interval 1.2-8.1). DISCUSSION: Although there was no overall increased risk of flare in the year after GAHT start, those with active IBD before hormone start and those who received testosterone were more likely to experience an IBD flare.