Background: Fatigue is being increasingly recognized as an important factor in surgical procedures, because it leads to errors in surgeons' decisions and reduced operating efficiency. In comparison to laparoscopic surgery, robot-assisted surgery is expected to reduce surgeon fatigue due to ergonomic adjustments, comfortable sitting positions, minimal restriction of instrument movement, and stable visualization. The aim of this study is to assess surgeon fatigue and explore the superiority of robot-assisted gastrectomy over laparoscopic gastrectomy in surgeon fatigue. Methods: This study is conducted as a prospective observational study ancillary to a randomized controlled phase III trial investigating the superiority in safety of robot-assisted gastrectomy over laparoscopic gastrectomy for clinical stage T1-4aN0-3 gastric cancer (JCOG1907: UMIN000039825). The lead surgeon and the first assistant performing surgeries as protocol treatment in JCOG1907 were included. Surgeon fatigue is quantitatively assessed using multiple assessment tools. The primary endpoint is the average NASA-Task Load Index (NASA-TLX) value for the lead surgeons. Secondary endpoints include six individual parameters of NASA-TLX, visual analog scale assessments for fatigue, advanced trail-making tests, and continuous heart rate variability measurements for lead surgeons and first assistants. The primary analysis compares surgeon fatigue between robot-assisted and laparoscopic surgery. Secondary analyses examine the association between surgeon fatigue and various factors, such as surgeon and patient background, and surgical findings, as well as cross-correlations among measures of surgical fatigue. In addition, the association between surgeon fatigue and postoperative complications and its impact on long-term survival outcomes is also evaluated. The sample size is 500 surgical cases, with a planned accrual period of 2 years. This study was initiated in April 2023 and registered at the University Hospital Medical Information Network as UMIN000052120. As of August 2023, 52 cases (10.4% of the planned accrual) have been enrolled in the study. Clinical trial information: UMIN000052120.