Background and Aims: The association between cancer treatments and exacerbation of inflammatory bowel diseases [IBD] is unclear. We aimed to evaluate the effects of cancer treatments on the disease activity of IBD. Methods: We performed a systematic review of the literature on cancer therapy in patients with pre-existing IBD. Electronic searches of PubMed, Cochrane Library and Embase were combined with manual searches (September 2021). Meta-analysis was performed using the random-effects model. The primary outcome was flares of IBD following cancer therapy. Secondary outcomes were need for IBD-related hospitalization, surgery, and initiation or intensification of steroid or biological treatments to manage IBD flares. Results: In total, 33 studies were included in the systematic review, comprising 1298 patients with IBD who received cancer treatment. The overall occurrence of IBD flares following cancer treatment was 30% (95% confidence interval [CI] 23-37%). IBD flares resulted in utilization of systemic steroids and biologic therapies among 25% and 10% of patients, respectively, and in discontinuation of cancer treatment among 14% of patients. Finally, the risk of gastrointestinal toxicity following immune check point inhibitor treatment [ICI] was increased in patients with IBD compared to patients without IBD (RR = 3.62 [95% CI 2.57-5.09]). Despite this, the studies generally reported that flares were manageable. Conclusions: Current data indicate a high proportion of patients with IBD experiencing a flare following the start of cancer treatment. Patients with IBD were at an increased risk of gastrointestinal toxicity following ICI treatment compared to those without IBD. However, cancer therapy-induced IBD flares were manageable and should not preclude appropriate cancer treatments.