Background: Currently no HER2 directed therapy is available in the perioperative setting for gastric and gastroesophageal junction (GEJ) cancer. Trastuzumab deruxtecan (T-DXd) received regulatory approval based on the encouraging efficacy demonstrated in the DESTINY-Gastric01 and DESTINY-Gastric02 studies in pretreated patients (pts) with metastatic gastric and GEJ cancer. This multicenter phase 2 study aimed to evaluate the clinical activity and safety of neoadjuvant T-DXd for locally advanced gastric and GEJ cancer. Methods: Eligible pts possessed previously untreated locally advanced gastric and GEJ adenocarcinoma with clinical stage of T2-4 and/or N+ without distant metastasis. The main cohort enrolled pts exhibiting HER2-positivity, defined as IHC 3+ or IHC 2+ with ISH+ by local assessment. An exploratory cohort included patients with HER2-low expression (IHC1+ or 2+ with ISH negative) with serum HER2-ECD exceeded 11.6 ng/ml. Treatment included three cycles of T-DXd administered every 3 weeks, followed by surgery. The primary endpoint was the major pathological response (MPR) rate by central assessment, with an expected rate of 45% and a futility threshold of 20%. The planned sample size in the main cohort was 27 pts with one-sided alpha of 10% and power of 90%, while an additional 10 pts would be enrolled into the exploratory cohort. Pre- and post-treatment samples were subjected to biomarker analyses. Clinical trial identification: NCT05034887. Results: Between November 2021 and November 2022, 27 pts were enrolled into the main cohort from seven Japanese sites. The majority of the pts had IHC3+ (24pts). The primary sites were gastric (16 pts) and GEJ (11 pts). Clinical stages ranged from II/III/IVa: 2/21/4 pts. Of the cohort, 26 pts completed planned 3 courses of T-DXd and one pt discontinued because of toxicity. R0 resection was achieved in 25 pts, with one undergoing R1 resection. The MPR rate as the primary endpoint was 14.8% (80% CI 6.6 % - 27.5 %) which did not surpass the predefined 20%. The pCR rate was 3.7% (95% CI 0.1 % - 19.0 %). There were no new safety signals during T-DXd treatment and during the post-surgery phase. Biomarker analyses remain ongoing. Conclusions: T-DXd monotherapy showed modest single agent activity for locally advanced HER2-positive gastric or GEJ adenocarcinoma in this phase 2 study. An additional cohort combining perioperative T-DXd with capecitabine and durvalumab is planned, to assess whether treatment efficacy and outcomes can be enhanced. Clinical trial information: NCT05034887.