Background: PD-1/L1 pathway inhibitors are effective in clear cell (cc)RCC, but efficacy of PD-1 inhibitors (or any therapy) in nccRCC has not been established. KEYNOTE-427 is a single-arm, open-label, phase 2 study of pembro monotherapy in patients (pts) with advanced ccRCC (cohort A) and nccRCC (cohort B). Cohort B results are presented. Methods: 165 pts with histologically confirmed nccRCC, no prior systemic therapy, measurable disease (RECIST v1.1), and KPS ≥70% enrolled. Pts received pembro 200 mg IV Q3W for 35 cycles (~2 y) or until progressive disease (PD), unacceptable toxicity, or withdrawal. Pts were followed after PD for overall survival. Primary end point: objective response rate (ORR) per RECIST v1.1 by blinded independent central review. Secondary end points included duration of response (DOR) and population description by International Metastatic RCC Database Consortium (IMDC) risk. Exploratory end points: ORR by histology and PD-L1 expression (combined positive score [CPS] ≥1 for PD-L1+). Results: Histology was confirmed by a central pathologist: papillary 72% (n=118), chromophobe 13% (n=21), unclassified 16% (n=26). 68% of patients were at intermediate/poor IMDC risk, and 62% were PD-L1+. At analysis, 49 pts had died and 3 had withdrawn. At a median follow-up duration of 11.1 mo (range, 0.9-21.3), 56% of pts discontinued pembro due to PD or clinical progression. Overall ORR was 24.8% (95% CI, 18.5-32.2; 8 [4.8%] CRs, 33 [20%] PRs); median DOR was not reached. ORR (95% CI) was 25.4% (17.9-34.3) with papillary, 9.5% (1.2-30.4) with chromophobe, and 34.6% (17.2-55.7) with unclassified nccRCC. ORR (95% CI) was 28.3% (16.8-42.3) with favorable and 23.2% (15.8-32.1) with intermediate/poor IMDC risk and 33.3% (24.3-43.4) and 10.3% (3.9-21.2) with CPS≥1 and CPS<1, respectively. Grade 3-5 treatment-related adverse events (TRAEs) occurred in 11% of pts; 6% discontinued due to TRAEs. 6 pts died due to AEs, 2 of which were TRAEs (pneumonia and cardiac arrest). Conclusions: Single-agent pembro showed encouraging antitumor activity in nccRCC, especially with papillary or unclassified histology. Safety profile of pembro was generally as expected. Clinical trial information: NCT02853344