Background: RC48-ADC is an antibody-drug conjugate (ADC) drug comprised of a novel humanized anti-HER2 IgG1, a linker, and a microtubule inhibitor, MMAE. The MoA included inhibition of HER2 signal pathway and cytotoxicity of MMAE. RC48-ADC has demonstrated promising anti-tumor activity in pre-clinical and early clinical studies. The current study is designed to evaluate the efficacy and safety of RC48-ADC in heavily treated patients with HER2-overexpressing (IHC 2+ or 3+) gastric or gastro-esophageal junction cancers. Methods: This is an open-label, multicenter, single-arm, phase II study. Eligibility criteria include: histologically confirmed gastric or gastro-esophageal junction cancers, HER2-overexpression (IHC 2+ or 3+), ECOG PS 0-1, post-to ≥2 prior systemic treatment. The patients received RC48-ADC, 2.5 mg/kg, q2w until disease progression, unacceptable toxicity, withdrawal, or study termination. The primary endpoint was ORR. PFS, OS, and safety were also evaluated. Results: Patient enrollment started in July 2017, and completed in November 2019. By the data cut-off date on 17-Dec-2019, 127 patients were enrolled. The median age was 58 years. At baseline, 59 patients (46.5%) had received ≥ 3 lines prior treatment. For the overall 127 patients, the investigator-assessed confirmed ORR was 18.1% (95% CI: 11.8%, 25.9%). Sub-group ORR was 19.4% and 16.9% for the patients post to 2 lines and ≥ 3 lines, respectively. For the 111 patients who were monitored for ≥ 2 cycles of efficacy assessments (i.e. 12 weeks), the ORR was 20.7% (95% CI: 13.6%, 29.5%). For the 127 patients, the mPFS was 3.8 months (95% CI: 2.7, 4.0, 89 events [70.1%]) and the mOS was 7.6 months (95% CI: 6.6, 9.2, 52 events [40.9%]). The most commonly reported treatment-related AEs were leukopenia (52.0%), alopecia (51.2%), neutropenia (48.0%), and fatigue (42.5%). Conclusions: RC48-ADC demonstrated a clinically meaningful response and survival benefit in the heavily treated patients with HER2-overexpressing gastric or gastro-esophageal junction cancers. The safety profile was in line with the previously reported data of RC48-ADC. RC48-ADC showed positive benefit/risk ratio for the target population. Clinical trial information: NCT03556345.