Background: Proximal gastrectomy (PG) has become an increasingly preferred procedure for treating early cancer in the upper third of the stomach when oncological safety is ensured, with expectation for improvement of nutritional status and quality of life (QOL) attributable to the remnant stomach. Although few studies compare postoperative outcomes of patients who undergo total gastrectomy (TG) and PG based on questionnaires, their significant limitations include the retrospective design, use of a questionnaire not specific to a postgastrectomy setting, and observation at a fixed time or after a short postoperative period. Methods: A multi-institutional prospective study (CCOG1602) of patients who undergo PG or TG for cStage I gastric cancer was conducted. We used the Post-Gastrectomy Syndrome Assessment Scale (PGSAS)-37 and The European Organization for Research and Treatment of Cancer quality of life questionnaire (EORTC-QLQ-C30) to evaluate the changes in body weight and QOL over a 3-year postoperative period. The primary endpoint was the weight loss rate 3 years after surgery. Results: We enrolled 109 patients from 18 institutions and selected 65 and 19 patients for inclusion in the TG and PG groups, respectively. There were no differences in age, sex, or preoperative body mass index between the TG and PG groups. Mean postoperative weight loss rates were 16.0% and 11.7% for the TG and PG groups, respectively (Cohen's D 0.656) during postoperative year 1 and 15.0% and 10.8% for TG and PG (Cohen's D 0.543), respectively, during postoperative year 3, indicating that the PG group achieved a better trend with a moderate effect size. According to the PGSAS-37, the PG group experienced a better trend in the indigestion subscale (Cohen's D –1.085) and total symptom score (Cohen's D –0.59) during postoperative year 3 compared with the TG group. In contrast, the EORTC-QLQ-C30 identified no difference between the groups at any time point during the 3-year postoperative period. Conclusions: The CCOG1602 demonstrates that PG tended to be more favorable compared with TG with respect to postoperative weight loss and QOL, particularly regarding indigestion. Clinical trial information: UMIN000020070.