Background Inflammatory bowel disease (IBD) and periodontitis are chronic, progressive, inflammatory diseases with similarly complex pathogeneses that involve an interplay between dysbiotic microbiota and dysregulated immune-inflammatory responses. However, whether the presence of periodontitis is associated with IBD activity and/or its severity remains unknown. Methods An online, questionnaire-based study was answered by 1093 patients with IBD, comprising 527 patients with Crohn's disease and 566 patients with ulcerative colitis. The survey included questions on social demographics; oral health, including the Periodontal Screening Score (PESS); and IBD-related characteristics, including validated disease indices. Results Irrespective of disease subtype, patients with a reduced number of teeth and those with self-reported severe periodontitis scored significantly higher on the IBD disability index (number of teeth: coefficient, 4.93 [95% confidence interval {CI}, 1.21-8.66; P = .010]; periodontitis: coefficient, 3.54 [95% CI, 0.27-6.80; P = .034]) and reported increased disease activity in the preceding 12 months (number of teeth: odds ratio [OR], 1.91 [95% CI, 1.36-2.69; P < .001]; periodontitis: OR, 1.71 [95% CI, 1.27-2.31; P < .001]). There was also evidence of a weak association between self-reported severe periodontitis and current disease activity (OR, 1.33; 95% CI, 0.95-1.86; P = .099). However, IBD severity, as a composite parameter of a history of surgery due to IBD and/or treatment with biological therapy, was not associated with possessing a reduced number of teeth (OR, 1.18; 95% CI, 0.77-1.80; P = .451), nor with self-reported severe periodontitis (OR, 1.15; 95% CI, 0.79-1.66; P = .467). Conclusions Periodontitis and tooth loss were significantly associated with increased IBD-related disability and more disease activity in the preceding 12 months. Our results suggest that greater attention should be paid to IBD patients' oral health. Lay Summary In this questionnaire-based study among 1093 patients with inflammatory bowel disease (IBD), we demonstrated a significant association between the presence of periodontitis and more IBD disease activity in the last 12 months, as well as increased IBD disability.