Background: High numbers of sTILs are predictive of pCR in TNBCs. This study examines the role of sTILs as a predictive biomarker for carboplatin (Cb) based NACT. Methods: sTILs were scored on 88 pre NACT TNBC biopsies as per the International TILs Working Group and were scored as continuous and categorical variables: 0-10%; 11-25%; 26-49%; ≥50%. OR was calculated using non-pCR as baseline parameter. Results: Patients with sTILs > 10% had increased pCR rates compared to those with sTILs ≤10%: pCR breast (B) (61% vs 31% p = 0.004); pCR breast/axilla (BA) (52% vs 29% p = 0.021). In those treated with Cb, there were no differences in pCR B between sTILs > 10% or ≤10% (67% vs 53%; p = 0.355) or pCR BA (60% vs 53%; p = 0.500). In those who did not receive Cb, sTILs > 10% had increased pCR B (57% vs 15%; p = 0.002) and pCR BA (46% vs 12%; p = 0.005) compared to sTILs ≤10%. Similar trends were seen with sTILs > 25% and ≥50%: differences were seen in patients who did not receive Cb, but not in those treated with Cb. In LPBCs, the addition of Cb did not significantly increase pCR rates: 67% vs 83% for Cb and non-Cb regimens (p = 0.583). In non-LPBC, pCR rates were increased with Cb: pCR B 59% vs 31% (p = 0.017), and pCR BA 56% vs 23% (p = 0.005) for Cb and non-Cb regimens. The association between sTILs and pCR was significant in multivariable models adjusting for grade and Cb. In patients who did not receive Cb (n = 54), increasing sTILs were associated with increased pCR B (OR 0.15 p = 0.004; OR 0.27 p = 0.039; OR 0.10 p = 0.05 for > 10%, > 25%, ≥50%) and pCR BA (OR 0.18 p = 0.019; OR 0.08 p = 0.029 for > 10%, ≥50%). In patients treated with Cb (n = 30), increasing sTILs were not associated with pCR B (OR 1.97 p = 0.581; OR 0.27 p = 0.334; OR 0.50 p = 0.719) or pCR BA (OR 2.93 p = 0.372; OR 0.79 p = 0.831; OR 0.40 p = 0.622 for > 10%, > 25%, ≥50%). Conclusions: sTILs are predictive of pCR: as sTILs increase, pCR rates increase. Tumors with high sTILs had high pCR rates to anthracycline-taxane (AT) NACT. The effect of increasing sTILs on pCR was most notable in patients who did not receive Cb based NACT, suggesting that tumors with high sTILs are inherently sensitive to AT based chemotherapy. Intensifying NACT with Cb could be used to increase pCR rates in patients with low sTILs. sTILs should be explored as a biomarker to intensify chemotherapy in those with low sTILs, and to de-escalate chemotherapy in tumors with high sTILs.