Background <p>Women diagnosed with a secondary triple negative breast cancer (TNBC) have worse overall survival (OS) and breast cancer-specific survival (BCSS) than women with a primary TNBC. There are currently no guidelines for treating a secondary TNBC. Therefore, we aimed to identify the treatments used for secondary TNBCs and evaluate them for associated survival differences.</p> Methods <p>This was a population-based, retrospective cohort study of premenopausal women diagnosed with a primary TNBC (N=9220) or secondary TNBC (N=682) from 2003 to 2019. We compared treatments for primary and secondary TNBC using multivariable logistic regression, with results presented as adjusted odds ratios and 95% confidence intervals (CIs). We examined OS and BCSS for primary versus secondary TNBC with multivariable Cox proportional hazards regression models, and results are presented as adjusted hazard ratios (HRs) and CIs.</p> Results <p>Secondary TNBCs were more commonly treated with no chemotherapy (27.3%) or a non-anthracycline (vs anthracycline)-based regimen, with taxotere + cyclophosphamide (TC, 23.8%) being most common. Women with a secondary TNBC were more likely to undergo a mastectomy (vs lumpectomy) than women with a primary TNBC (odds ratio 2.01 [95% CI 1.12–2.55]). Women with secondary (vs primary) TNBC had worse OS with no chemotherapy (HR 1.56 [95% CI 1.09–2.23]) and TC (HR 1.69 [95% CI 1.12–2.55]), but no significant differences in BCSS were seen with treatment type.</p> Conclusion <p>Some differences in treatments used for secondary TNBC may be associated with worse OS, but patients still experienced worse BCSS despite similar treatments. Our findings suggest that more aggressive treatment or consideration for chemotherapy at earlier stage disease is warranted.</p>