Background: We evaluated the safety and efficacy of adding trastuzumab to ramucirumab and paclitaxel (TRP) as a second line treatment in human epidermal growth factor receptor 2 (HER-2)-positive advanced gastric or gastroesophageal junction (G/GEJ) cancer progressed from trastuzumab containing chemotherapy. Methods: Patients with HER-2-positive advanced G/GEJ cancer who progressed after first-line chemotherapy with trastuzumab in combination with fluoropyrimidine and platinum were eligible. Trastuzumab (Herzuma[CT-P6], Celltrion Inc.) 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: At the phase 1b part, as there was no dose limiting toxicity in 3 patients at the dose level 1, dose level 1 with full dose combination was determined as recommended phase 2 dose. At the time of data lock on Jan. 31, 2021, 45 patients among enrolled 50 patients were evaluable for response and safety including 3 patients from phase 1b part. Median age was 59 years old (range 30-82) and most patients were male (37/45). At baseline, 33 patients had tumors with HER-2 3+ by immunohistochemistry (IHC) and 12 had those with HER-2 2+ by IHC with ERBB2 amplification by in situ hybridization. With median follow-up duration of 11.6 months, median PFS and OS were 7.2 months (95% confidence interval [CI]: 6.0-8.5 months) and 13.6 months (95% CI: 10.3-16.9 months), respectively. ORR was 55.6% (25/45, complete response = 1, partial response = 24) and DCR was 95.6% (43/45), respectively. Most common hematologic adverse event (AE) was neutropenia (all grade: 64.4%, grade 3/4: 51.1%) with 1 case of febrile neutropenia (2.2%). Most common non-hematologic AE was peripheral sensory neuropathy (all grade: 33.3%, grade 3: 2.2%). Gastrointestinal (GI) bleeding occurred in 4 patients (grade 3 upper GI bleeding: 6.7%, grade 1 lower GI bleeding: 2.2%), whereas GI perforation was not observed. Hypertension occurred in 3 patients (all grade: 6.7%, grade 3: 4.4%). No new or unexpected AEs resulting in treatment cessation were observed with this combination regimen. Conclusions: The continuous use of trastuzumab beyond progression 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. Updated outcomes for ongoing patients will be presented.