This single-arm, open-label study will evaluate the safety and efficacy of the combination oxaliplatin, capecitabine, and trastuzumab with chemoradiotherapy in the adjuvant setting in participants with curatively resected HER2+ gastric or gastroesophageal junction cancer. Participants will receive trastuzumab 8 milligrams per kilogram (mg/kg) intravenously (IV) on Day 1 of Cycle 1 and 6 mg/kg IV on Day 1 of every following 3-week cycle, with oxaliplatin 100 milligrams per square meter (mg/m^2) IV on Day 1 of Cycles 1-3, and capecitabine 850 mg/m^2 orally twice daily on Days 1-14 of Cycles 1-3 and on 5 days per week during chemoradiotherapy. Radiotherapy will be given at a total dose of 45 gray (Gy) divided into 25 doses on 5 treatment days each week for 5 weeks starting Day 22 of Cycle 3. Anticipated time on study treatment is 1 year plus a 1-year follow-up period.

Inclusion Criteria:

• Curatively resected HER2+ gastric or gastroesophageal junction adenocarcinoma; HER2+ status as defined by immunohistochemistry-2 positive or 3 positive with corroborative Fluorescence In Situ Hybridization+ result
• Participants with stages between Stage IB (T1N1M0) and Stage IIIC
• ECOG performance status score equal to or less than (<=) 2 during screening
• Left ventricular ejection fraction equal to or higher than (>=) 55% with acceptable levels of liver and renal functions
• No known contraindication to capecitabine, oxaliplatin, and trastuzumab
• No contraindication for radiotherapy or has not received any previous radiotherapy to the gastric region for any reason

Exclusion Criteria:

• Participants with a malign condition in the last 5 years except squamous cell carcinoma of the skin
• Previous neoadjuvant chemotherapy and/or radiotherapy
• Any disruption in the physical integrity of the upper gastrointestinal tract (except surgical intervention for gastric or gastroesophageal junction carcinoma)
• Known (previously diagnosed and ongoing) malabsorption syndrome
• Active gastrointestinal bleeding
• Participants with Stage IV gastric or gastroesophageal junction adenocarcinoma
• Clinically significant cardiac or cardiovascular disease
• Uncontrolled hypertension
• Participants who have received any investigational anti-cancer treatment or are being treated in a concomitant investigational drug study
• Abnormal laboratory values at screening for serum total bilirubin, alanine aminotransferase or aspartate amino transferase, alkaline phosphatase, absolute neutrophil count, platelet count, and/or hemoglobin
• Known or suspected hypersensitivity against trastuzumab or proteins of rodents
• Pregnancy or lactation