Background: Inflammation is important in multiple myeloma pathogenesis, and regular aspirin use has been shown to confer a reduced risk of multiple myeloma. The influence of aspirin on survival after multiple myeloma diagnosis is unknown. Methods: We identified 436 men and women diagnosed with multiple myeloma between 1980 and 2016 in the Health Professionals Follow-up Study and the Nurses’ Health Study who reported aspirin intake biennially on follow-up questionnaires. Using multivariable Cox proportional hazards regression models, we estimated HRs and 95% confidence intervals (CI) associated with the effect of aspirin use on multiple myeloma–specific and overall mortality. Results: Compared with nonusers, participants who used aspirin after diagnosis had a multivariable HR for multiple myeloma–specific mortality of 0.61 (95% CI, 0.46–0.79) and for overall mortality of 0.63 (95% CI, 0.49–0.80), after adjustment for age at diagnosis, year of diagnosis, sex, body mass index, prediagnosis aspirin use, and number of comorbidities. For postdiagnosis aspirin quantity, we observed a modest trend of reduction in multiple myeloma–specific and all-cause mortality with increasing number of 325-mg tablets of aspirin per week, although the CIs for 1 to <6 and ≥6 tablets overlapped. Results were not materially different before or after the availability of novel therapies (before vs. after the year 2000). Prediagnosis frequency or duration of aspirin use was not significantly associated with multiple myeloma–specific or overall mortality. Conclusions: These findings support the use of aspirin as a complementary strategy to enhance multiple myeloma survival. Impact: Confirmation in samples that have comprehensive clinical information is encouraged.