Background: The purpose of this phase I trial was to evaluate the safety and toxicity of preoperative short-course chemoradiotherapy (CXRT) in patients with potentially resectable gastric or gastroesophageal adenocarcinoma as part of a total neoadjuvant therapy (TNT) approach. Methods: Patients with potentially resectable gastric or gastroesophageal adenocarcinoma received CXRT consisting of 30 Gy in 10 fractions with concurrent capecitabine or fluorouracil, followed by systemic therapy for 2 months and then surgery. The primary objective was safety of CXRT with secondary endpoints of pathologic complete response, perioperative complications, and overall survival (OS). Results: A total of 24 patients were treated between March 2021 and December 2022. Ten patients (42%) presented with bleeding and 3 (13%) presented with gastric outlet obstruction. Two patients (8%) had cirrhosis. Clinical nodal involvement was reported in 12 patients (50%). Twenty patients (83%) had poorly differentiated tumors, and 13 (54%) had signet ring cell histology. All patients completed CXRT. CXRT treatment-related toxic effects included grade 3 lymphopenia in 7 patients (29%), grade 4 lymphopenia in 1 (4%), and grade 3 anemia in 1 (4%). After CXRT, 22 patients (92%) completed 2 months of chemotherapy, 1 patient (4%) with a microsatellite-high tumor completed immunotherapy, and 1 patient (4%) underwent resection without systemic therapy. All patients underwent attempted resection, and gastrectomy was performed in 20 (83%). R0 resection rate was 95%. Pathologic complete response was observed in 2 patients (8%), and ≤ 1% viability was observed in an additional 4 (17%). Three patients had surgical complications (grade I in 1 patient [4%], grade IIIb in 1 [4%], and grade IVa in 1 [4%]); there were no deaths within 90 days. The median follow-up time was 20 months, and median OS was not reached. One- and 3-year OS rates from the date of diagnosis were 96% and 80%, respectively. Conclusions: Short-course CXRT (30 Gy in 10 fractions) may be safely used as part of a planned TNT approach. Rates of treatment completion, pathologic response, and survival were promising, supporting further research into TNT for gastric cancer. Clinical trial information: NCT04523818.