Background: HER-RAM study is the phase Ib/II multicenter study to evaluate the safety and efficacy of adding trastuzumab to ramucirumab and paclitaxel as a second-line treatment in HER2-positive advanced gastric or gastroesophageal junction (G/GEJ) cancer progressed from trastuzumab containing chemotherapy. We report survival follow up and exploratory biomarker results. Methods: Patients with HER2-positive advanced G/GEJ cancer who progressed after first-line trastuzumab containing chemotherapy were enrolled. Trastuzumab 4mg/kg on day 1 followed by 2mg/kg on days 8, 15, and 22, ramucirumab 8mg/kg on days 1 and 15, and paclitaxel (dose level 1: 80mg/m², dose level -1: 70 mg/m²) on days 1, 8, and 15 of a 28-day cycle was tested. After safety analysis of lead-in safety cohort (phase 1b), phase 2 part was conducted to evaluate the primary endpoint of progression-free survival (PFS). Secondary endpoints included objective response rate (ORR), disease control rate (DCR), overall survival (OS), and safety. Results: No dose limiting toxicity at the dose level 1 was observed at the phase Ib part, the dose level 1 with full dose combination was determined as recommended phase 2 dose. At the time of data lock on September 7th, 2021, 50 patients including 3 patients from the phase 1b part were evaluable for response and safety. Median age was 60 years old (range 29-82) and most patients were male (40/50). At baseline, 39 patients had tumors with HER-2 3+ by immunohistochemistry (IHC) and 11 had those with HER-2 2+ by IHC with ERBB2 amplification by in situ hybridization. With median follow-up duration of 18.8 months, median PFS and OS were 7.0 months (95% confidence interval [CI]: 4.9-9.2 months) and 13.6 months (95% CI: 9.5-17.6 months), respectively. ORR was 54% (27/50, complete response = 1, partial response = 26) and DCR was 96% (48/50), respectively. On the exploratory analysis, HER-2 positivity of tumor tissue was lost after fist-line chemotherapy in 8 of 23 patients (34.7%) without any definite association between loss of HER-2 and outcomes. Most common hematologic adverse event (AE) was neutropenia (all grade: 64%, grade 3/4: 52%) with 1 case of febrile neutropenia (2%). Most common non-hematologic AE was peripheral sensory neuropathy and anorexia (all grade: 32%, grade 3: 2%, respectively). Gastrointestinal (GI) bleeding occurred in 6 cases (grade 3 upper GI bleeding: 3 patients, grade 1/2 lower GI bleeding: 3 patients), whereas GI perforation was not observed. Hypertension occurred in 3 patients (grade 1/2: 1 patient, grade 3: 2 patients). No new or unexpected AEs were observed. Conclusions: The continuous use of trastuzumab in combination with ramucirumab and paclitaxel showed promising activity and manageable safety profile in HER2-positive G/GEJ cancer patients who progressed after trastuzumab containing chemotherapy. Clinical trial information: NCT04888663.