PURPOSE Epidermal growth factor receptor ( EGFR ) tyrosine kinase inhibitors (TKIs) are standard first-line therapy for EGFR -mutant, metastatic non–small cell lung cancer (NSCLC); however, most patients experience disease progression. We report results from the randomized, double-blind, phase III KEYNOTE-789 study of pemetrexed and platinum–based chemotherapy with or without pembrolizumab for TKI-resistant, EGFR -mutant, metastatic nonsquamous NSCLC (ClinicalTrials.gov identifier: NCT03515837 ). METHODS Adults with pathologically confirmed stage IV nonsquamous NSCLC, documented DEL19 or L858R EGFR mutation, and progression after EGFR-TKI treatment were randomly assigned 1:1 to 35 cycles of pembrolizumab 200 mg or placebo once every 3 weeks plus four cycles of pemetrexed and carboplatin or cisplatin once every 3 weeks and then maintenance pemetrexed. Dual primary end points were progression-free survival (PFS) and overall survival (OS). Final PFS testing was completed at the second interim analysis (IA2; data cutoff, December 3, 2021); OS was tested at final analysis (FA; data cutoff, January 17, 2023). Efficacy boundaries were one-sided P = .0117 for PFS and OS. RESULTS Four hundred ninety-two patients were randomly assigned to pembrolizumab plus chemotherapy (n = 245) or placebo plus chemotherapy (n = 247). At IA2, the median PFS was 5.6 months for pembrolizumab plus chemotherapy versus 5.5 months for placebo plus chemotherapy (hazard ratio [HR], 0.80 [95% CI, 0.65 to 0.97]; P = .0122). At FA, the median OS was 15.9 versus 14.7 months, respectively (HR, 0.84 [95% CI, 0.69 to 1.02]; P = .0362). Grade ≥3 treatment-related adverse events occurred in 43.7% of pembrolizumab plus chemotherapy recipients versus 38.6% of placebo plus chemotherapy recipients. CONCLUSION Addition of pembrolizumab to chemotherapy in patients with TKI-resistant, EGFR -mutant, metastatic nonsquamous NSCLC did not significantly prolong PFS or OS versus placebo plus chemotherapy in KEYNOTE-789.