Purpose: Laboratory studies have shown anti-neoplastic properties of non-aspirin NSAID; however, no studies have examined the influence of non-aspirin NSAIDs as potential adjuvant cancer therapy in women with endometrial cancer. We therefore examined the association between post-diagnostic use of non-aspirin NSAIDs and endometrial cancer mortality in Denmark. Methods: We identified all women with a primary endometrial cancer diagnosis between 2000 and 2012, who were alive one year after the diagnosis. Information on drug use, cause-specific mortality and potential confounders was obtained from nationwide health- and demographic registries. Cox regression models were used to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for the association between post-diagnostic non-aspirin NSAID use and endometrial cancer mortality. Results: Among 6 694 endometrial cancer patients with a maximum follow-up of 13 years, 753 women died from endometrial cancer. Post-diagnostic non-aspirin NSAID use (≥ 1 filled prescription) was associated with an overall HR of 1.15 (95% CI; 0.97–1.36) for endometrial cancer mortality, with higher HRs for the highest intensity of use (HR; 1.40, 95% CI; 1.11–1.77) and largest cumulative amount (HR; 1.56, 95% CI; 1.14–2.14). Conclusion: Our findings yielded no evidence that use of non-aspirin NSAIDs was associated with reduced endometrial cancer. Rather, we observed that high-intensity and large cumulative amount of non-aspirin NSAID use may be associated with increased endometrial cancer mortality.