Background: Accumulating evidence suggests that aspirin use may improve survival in cancer patients, however, for endometrial cancer, epidemiological evidence is limited and results are equivocal. In a nationwide cohort study, we examined the association between post-diagnostic low-dose aspirin use and endometrial cancer mortality. Methods: From the Danish Cancer Registry, we identified all women with a primary diagnosis of endometrial cancer. Women diagnosed between 2000 and 2012, aged 30-84 years, who had no history of cancer (except non-melanoma skin cancer) and were alive 1 year after the cancer diagnosis were eligible. We obtained information on pre- and post-diagnostic use (≥1 prescription) of low-dose aspirin, mortality and potential confounding factors from nationwide registries. Using Cox regression models, we estimated adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for the association between post-diagnostic low-dose aspirin use and endometrial cancer mortality. The exposure was modelled as both time-varying as well as time-fixed within exposure windows of 1 and 5 years. Results: We identified 6694 endometrial cancer patients with a maximum follow-up of 13 years. In the time-varying analysis, post-diagnostic low-dose aspirin use was associated with a HR of 1.10 (95% CI 0.90-1.33) for endometrial cancer mortality. We found no indication of a dose-response association according to increasing tablet strength, cumulative amount or duration of use, and the HRs were similar for pre-diagnostic and post-diagnostic low-dose aspirin use compared with non-use. Conclusions: We found no indication that post-diagnostic low-dose aspirin use was associated with reduced mortality for endometrial cancer; rather our findings suggested a concern for increased mortality.