Background: The impact of gut microbiome on tumor biology, progression and response to immunotherapy has been shown across cancer types. However, there is little known about the impact of gut microbial composition on response to chemotherapy. We have previously shown that the gut microbiome remains unaltered during NACT in a cohort of 32 patients. Here we investigate the association between gut microbiome and response to NACT in a larger cohort of early-stage TNBC. Methods: Longitudinal fecal samples were collected from 85 patients with newly-diagnosed, early-stage TNBC patients enrolled in the ARTEMIS trial (NCT02276443). Patients all received standard NACT with adriamycin/cyclophosphamide (AC); volumetric change was assessed using ultrasound and patients with < 70% volumetric reduction (VR) after 4 cycles of AC were recommended to receive targeted therapy in addition to standard NACT to improve response rates. We performed 16S sequencing on bacterial genomic DNA extracted from 85 pre-AC fecal samples using the 2x250 bp paired-end read protocol. Quality-filtered sequences were clustered into Operational Taxonomic Units and classified using Mothur method with the Silva database version 138. For differential taxa-based univariate analysis, abundant microbiome taxa at species, genus, family, class, and order levels were analyzed using DESeq2 after logit transformation. Alpha-diversity indices within group categories were calculated using phyloseq. Microbial alpha diversity (within-sample diversity) was measured by Simpson's reciprocal index. β-diversity was measured using weighted UniFrac distances between the groups. The association between microbiota abundance and pathologic complete response (pCR) or residual disease (RD) was assessed using DESeq2 analysis. Results: Pre-AC fecal samples from 85 patients were available for analysis. Amongst them, there were 46 patients with pCR and 39 patients with RD. There was no significant difference in alpha diversity (p = 0.8) or beta-diversity (p = 0.7) between the pCR and RD groups. However, relative to patients with RD, the gut microbiome in patients with pCR was enriched for the Bifidobacterium longum species (p = 0.03). The gut microbiome in patients with RD was enriched for Lachnospiraceae (p = 0.03) at the genus level and the Bacteroides thetaiotaomicron species (p = 0.02). Conclusions: We have demonstrated significant differences in the gut microbial composition in patients with pCR as compared to patients with RD. Further investigation in larger studies is needed to support therapeutic exploration of gut microbiome modulation in TNBC patients receiving chemotherapy such as probiotic supplementation or fecal microbiota transplant.