Introduction: Immune checkpoint inhibitors (ICI) lead to immune activation, increased inflammation and cancer cell death. Both immune activation and inflammation are critical pathobiological drivers for venous thromboembolism (VTE). There are no robust data testing the effect of ICIs on the risk of developing VTE. Methods: This is a retrospective study of 2854 patients who received ICIs at Massachusetts General Hospital, Boston, MA. VTE events, defined as a composite of deep vein thrombosis (DVT) or pulmonary embolism (PE), were identified by individual chart review and were blindly adjudicated using standard criteria. A case-crossover design was applied with an “at-risk period” defined as the two-year period after and the “control period” as the two years prior to treatment. Incidence rate ratio (IRR) was calculated using Poisson’s regression. Results: Immune checkpoint inhibitor use increased VTE risk by 1.84-fold from 4.85 per 100-person years to 8.91 per 100 person-years (IRR 1.84, 95% confidence interval: 1.54 - 2.19, p <0.001). Of the individual components, there was a 2.44-fold increase in DVT risk (2.30 to 5.58 per 100 person-years) and 1.68-fold increase in PE risk (2.96 to 5.00 per 100 person-years). Comparing patients with and without a VTE event, those with a VTE event after ICI initiation had a higher rate of prior VTE, lung cancer, urothelial cancer, and a higher platelet count and white blood cell count at baseline. At 6 months post ICI initiation, 165 (8.6%) patients had a VTE event and of these patients 136 (7.1%) had no prior VTE. Conclusions: Patients with cancer treated with ICIs are at increased risk of developing VTE. Whether prophylaxis for VTE among patients starting an ICI reduces this risk is unclear.