Background: Standard first-line regimens for patients with metastatic gastroesophageal adenocarcinomas have moderate clinical benefit with objective response rates (ORR) of approximately 40-50%. FOLFIRINOX has been shown to be an effective and well-tolerated first line therapy in other GI cancers. In this open-label, single-arm phase II study of patients with advanced gastroesophageal adenocarcinomas, we sought to evaluate the safety and clinical activity of FOLFIRINOX. Methods: The primary endpoint was ORR, and secondary endpoints included safety profile, progression free survival (PFS), overall survival (OS), time to progression (TTP), clinical benefit rate (CBR), and duration of response. Estimated sample size included 41 patients with HER2 negative disease with 90% power to detect an ORR≥60% with alpha of 0.10. No enrollment goal was planned for HER2 positive patients, but they were allowed participation to receive study treatment in combination with trastuzumab. Treatment consisted of 400mg/m2 5-FU bolus, 400 mg/m2 leucovorin, 2400 mg/m2 5-FU infusion over 46 hours, 180 mg/m2 irinotecan, and 85 mg/m2 oxaliplatin. Trastuzumab was administered intravenously as a 6 mg/kg loading dose then given 4 mg/kg every 14 days for HER2 positive patients. This trial is registered with ClinicalTrials.gov, NCT01928290. Results: From November 2013 to May 2019, 67 patients were enrolled, of which 26 (39%) had HER2 positive disease. Median follow-up was 16.1 months. ORR was 61% (25/41) for HER2 negative and 85% (22/26) for HER2 positive groups. Overall, one patient (2%) had a complete response, 36 patients (69%) had partial responses, and 13 patients (19%) had stable disease for >6 months; therefore, CBR was 96%. Median PFS was 11.9 months, median OS was 17.4 months. 41 patients (83.7%) had dose modification or treatment delay with the most common toxicities being neutropenia, diarrhea, peripheral sensory neuropathy, and nausea with no unexpected toxicities. Conclusions: FOLFIRINOX is a highly effective three-drug regimen for first-line treatment of advanced gastroesophageal cancer with expected, tolerable toxicities. Clinical trial information: NCT01928290.