BACKGROUND: There is a scarcity of data exploring the benefits of adjuvant or neoadjuvant chemotherapy in the treatment of breast cancer in older women. We aimed to explore the effect of adding chemotherapy to local therapy on overall survival in older women with triple-negative breast cancer. METHODS: For this propensity-matched analysis, we used data from the National Cancer Database, a joint project of the Commission on Cancer of the American College of Surgeons and the American Cancer Society. We included data from women aged 70 years or older with surgically treated, American Joint Committee on Cancer (AJCC) Stage I-III invasive triple-negative breast cancer diagnosed from 2004 to 2014. Patients with T1aN0M0 disease and those with incomplete data on oestrogen receptor status, progesterone receptor status, or HER2 status were excluded. To reduce bias, patients were subdivided into three groups: those who were recommended chemotherapy but did not receive it; those who received chemotherapy; and those for whom chemotherapy was not recommended and not given. The primary outcome was overall survival. Multivariate Cox regression analysis and propensity score matching were done to minimise bias. FINDINGS: Between Jan 1, 2004, and Dec, 31, 2014, 16 062 women with triple-negative breast cancer in the database met the inclusion criteria for this analysis. Median follow-up was 38·3 months (IQR 20·7-46·1, range 0-138·0; 95% CI 37·8-38·7). Collectively, the 5-year overall survival estimate of the 16 062 patients in the study cohort was 62·3% (95% CI 59·7-64·4). 5-year estimated overall survival was 68·5% (95% CI 66·4-70·6) for patients receiving chemotherapy, 61·1% (59·0-63·2) for patients recommended but not given chemotherapy, and 53·7% (51·8-55·8) for patients not recommended chemotherapy and not given chemotherapy (pooled log rank p<0·0001). Multivariate Cox regression analysis of a propensity score-matched sample comparing those who received chemotherapy with those who were recommended but not given chemotherapy (n=1884 matched pairs) identified improved overall survival with chemotherapy (hazard ratio [HR] 0·69 [95% CI 0·60-0·80]; p<0·0001). After stratifying the propensity score matching sample, this benefit persisted for node-negative women (HR 0·80 [95% CI 0·66-0·97]; p=0·007), node-positive women (0·76 [0·64-0·91]; p=0·006), and those with a comorbidity score greater than 0 (HR 0·74 [95% CI 0·59-0·94]; p=0·013). INTERPRETATION: These data support consideration of chemotherapy in the treatment of women aged 70 years or older with triple-negative breast cancer. FUNDING: None.