Background: To evaluate the safety and initial efficacy of adoptive cell therapy (ACT) combined with SOX in first-line treatment of locally advanced unresectable or metastatic gastric adenocarcinoma/gastroesophageal adenocarcinoma. Methods: In this single-arm, single-center exploratory trial, patients with histologically confirmed locally advanced unresectable or metastatic gastric adenocarcinoma/gastroesophageal adenocarcinoma were randomly assigned (1:1) to receive ACT in combination with S-1 and oxaliplatin (SOX) or SOX alone. The primary endpoint was the incidence of adverse events (AEs). Secondary endpoints were progression-free survival (PFS), overall survival (OS), objective response rate (ORR), and disease control rate (DCR). Results: Fifty-nine patients were enrolled in the study between November 20, 2014 and September 6, 2017. 31 patients received ACT combined with SOX (ACT-SOX) and 28 patients received SOX. The most common AEs in both groups were gastrointestinal reaction, leucopenia, neutropenia, anemia, thrombocytopenia, hyperbilirubinemia and elevated aspartate transaminase concentration, with a higher incidence in the SOX group. The median PFS for ACT-SOX and SOX were 6.9 and 4.9 months respectively (HR 0.80, p=0.45), and the median OS were 17.8 and 9.75 months (HR 0.76, p=0.34). Patients who received more than three injections of specific lymphocyte subsets benefited the most from combination therapy. Cox univariate and multivariate analysis showed that tumor metastasis to more than two organs was the main risk factor for PFS and OS. 29 patients in the ACT-SOX group and 25 in the SOX group had measurable lesions. The ORR of ACT-SOX group and SOX group was 55.2% and 32.0%, and DCR of two groups was 93.1% and 88.0%. Conclusions: The safety of ACT-SOX in first-line treatment of patients with locally advanced unresectable or metastatic gastric adenocarcinoma/gastroesophageal adenocarcinoma is good. Compared with SOX alone, although the PFS and OS of ACT-SOX did not reach statistical difference, it could further extend the time of PFS and OS, and the absolute improvement time of OS was about 8.05 months. Continuous benefit of ACT-SOX was observed through long-term follow-up. Clinical trial information: NCT02504229.