Background: The role and effect of radiotherapy in the development of VTE has not been extensively explored; Methods: This is a post-hoc analysis from the COMPASS-CAT trial. Patients with breast, lung, colon or ovarian cancer, with early, locally advanced or metastatic disease and receiving chemotherapy were included. Primary endpoint was documented symptomatic VTE; Results: A total of 1355 patients were enrolled between November 2013 and November 2015. Of those, 194 patients were excluded because of missing data or the use of anticoagulation. Of the evaluable patients, 361 patients received radiotherapy (33.6%) At a median follow up of 6 months, 9.1% (n = 33) of patients receiving radiotherapy developed a VTE event (excluding those with missing data on follow up). After applying the competing risk model, radiotherapy remained significantly associated with increased risk for VTE (HR 2.47, 95% CI: 1.47–4.12, p = 0.001). Stratification analysis for the cohort that received radiotherapy revealed an increased risk of VTE in women compared to men (10.8% vs. 2.7%; p = 0.03), in those older than 50 (12.2% vs. 3.7%; p = 0.011); for patients receiving anthracycline chemotherapy (14.4% vs. 2.9%; p < 0.001) and hormonal therapy (12.9% vs. 3.9%; p < 0.001); Conclusions: Analysis from the COMPASS-CAT revealed a significant correlation between radiotherapy and VTE in patients with cancer. Further studies are needed to better understand the potential cellular toxicity associated with radiotherapy. View Full-Text