Several recent observational studies have linked low-dose aspirin use to improved survival in patients with head and neck cancer. However, studies of patterns of aspirin use and risk of cancer-specific mortality are lacking. This nationwide cohort study included all patients in the Danish Cancer Registry with a primary diagnosis of head and neck squamous cell cancer (HNSCC) during 2000 to 2016, aged 30 to 84 years, without prior cancer (except nonmelanoma skin cancer) and alive 1 year after diagnosis. Nationwide registries provided information on filled prescriptions, mortality and potential confounding factors. For a subpopulation, a clinical database provided additional information, including human papillomavirus (HPV) tumor status. We used Cox proportional hazards regression models to estimate adjusted hazard ratios (HRs) with 95% confidence intervals (CIs) for the association between postdiagnostic low-dose aspirin use (≥1 prescription within first year after diagnosis) and risk of cancer-specific mortality. We identified 10 770 patients with HNSCC during a median follow-up of 3.9 years. Of these, 1799 (16.7%) were low-dose aspirin users. Postdiagnostic use of low-dose aspirin was associated with a HR of 0.97 (95% CI 0.82-1.15) for cancer-specific mortality. Similar neutral associations were found according to patterns of aspirin use. No apparent trends emerged according to age, sex, topography or stage. A tendency towards a decreased cancer-specific mortality risk with low-dose aspirin use was observed among HPV-positive patients; however, the statistical precision was low. In conclusion, we did not observe an association between postdiagnostic low-dose aspirin use and cancer-specific mortality in a nationwide cohort of patients with HNSCC.