The association between peripheral blood T-cell receptor (TCR) repertoire and chemotherapy response remains unclear. We evaluated peripheral blood TCR repertoire and gut microbiota in 21 breast cancer patients receiving neoadjuvant chemotherapy (NAC) who were enrolled in the SBP-14 prospective cohort study between October 2019 and March 2022. We analyzed stool samples obtained pre-NAC and peripheral blood samples obtained pre- and post-NAC. The primary endpoint was the association between baseline peripheral blood TCR repertoire and treatment response. Eleven patients (52%) had hormone receptor-positive breast cancer. All patients received anthracyclines and taxanes, with anti-HER2 therapy for HER2-positive disease. TCR diversity (TCR alpha chain [TRA], p=0.095; TCR beta chain [TRB], p=0.051) and clonality (TRA, p=0.271; TRB, p=0.500) were not significantly associated with treatment response. Gut microbiota diversity was not correlated with TCR diversity (TRA: ρ=0.046, p=0.845; TRB: ρ=0.021, p=0.929); however, three bacterial orders (Erysipelotrichales, Bacillales, and Pasteurellales) were significantly associated with TCR repertoire. Baseline TCR repertoire was not associated with treatment response in this cohort of breast cancer patients.