Background: In the phase Ib KEYNOTE-012 study, the anti–PD-1 antibody pembro demonstrated promising antitumor activity and acceptable safety as monotherapy in pretreated patients (pts) with PD-L1+ mTNBC. Adding pembro to chemo may enhance antitumor activity. KEYNOTE-355 (NCT02819518) is a global phase III study of pembro+chemo vs PBO+chemo in pts with previously untreated, locally recurrent, inoperable TNBC/mTNBC. Methods: Eligible pts are ≥18 y and have centrally confirmed, locally recurrent, inoperable TNBC or mTNBC not previously treated with chemo (prior chemo in [neo]adjuvant setting is allowed); measurable disease per RECIST v1.1; ECOG PS 0-1; ≥6 mo between definitive surgery or last dose of adjuvant chemo, whichever was last; and first disease recurrence (≥12 mo if prior treatment with same-class agent). Part 1 is an open-label, unblinded safety run-in of ~30 pts in 3 arms (pembro+nab-paclitaxel, pembro+paclitaxel, pembro+gemcitabine/carboplatin). Part 2 is a double-blind, PBO-controlled study of ~828 pts randomized 2:1 to pembro 200 mg every 3 weeks + chemo (nab-paclitaxel 100 mg/m² on d 1, 8, and 15 every 28 d; paclitaxel 90 mg/m² on d 1, 8, and 15 every 28 d; or gemcitabine 1000 mg/m²+ carboplatin AUC 2 on d 1 and 8 every 21 d) or PBO+chemo. Crossover is not allowed. Stratification factors are study chemo (taxane vs gemcitabine/carboplatin), tumor PD-L1 expression (+/-), and prior therapy with same-class agent in the (neo)adjuvant setting (yes/no). Treatment will occur for ≤35 administrations (pembro/PBO only) or until confirmed disease progression, unacceptable toxicity, withdrawal of consent, or decision to discontinue. Primary end points are safety in part 1 and PFS (by RECIST v1.1, central radiology review) and OS in part 2; secondary end points include ORR (by RECIST v1.1, central radiology review) and duration of response. AEs will be graded per NCI CTCAE v4.0. Response will be assessed at wk 8, 16, 24, then at 9-wk intervals up to 1 y, and at 12-wk intervals thereafter. Interim safety analysis will occur after pts complete 1 treatment cycle in part 1. Clinical trial information: NCT02819518