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Background Both inflammatory bowel disease (IBD) and ankylosing spondylitis (AS) are inflammatory diseases but there was little previous study that demonstrate the IBD severity and outcome in IBD patients with concomitant AS. Methods Patients’ records were collected from 3 tertiary hospitals (Seoul national university hospital, Seoul national university Bundang hospital, and Seoul metropolitan government Seoul national university Boramae medical center) from October 2004 to June 2021. Patients with IBD and concomitant AS (IBD-AS group) were identified and propensity score matching (PSM) was applied to match the IBD-AS group and IBD patients without AS (only IBD group). Results After PSM, significantly more patients in IBD-AS group had colectomies (p = .017) or were prescribed biologics (p < .001), immunosuppressive agents (p = .021), and steroid (p = .017) than in only IBD group. The number of patients treated with biologics (p < .001) or immunosuppressive agents (p = .032) were significantly greater in UC-AS group than in only UC group. There was no significant difference in outcomes between only CD group and CD-AS group. In logistic regression analyses, identified that concomitant AS was a significant factor associated with biologics treatment in patients with IBD. Kaplan-Meier analyses demonstrated that there was a significant difference in the probability of starting biologics treatment between IBD patients with and without concomitant AS (p = 0.002). In UC patients, the probability of starting biologics was also significantly different according to concomitance of AS (p < 0.001). Concomitant AS was a risk factor for predicting biologics treatment in patients with UC in Cox regression analysis after adjustment UC (adjusted hazard ratio, 6.296; confidence interval, 2.243 to 17.668; p < 0.001) Conclusion Patients with IBD-AS group were more likely to have a higher severity than in only IBD group. The current study result can help IBD specialists understand and treat patients with IBD and concomitant AS better.