Background: This study evaluated the residual gastric volume (GV) using ultrasonog-raphy in fasting older adults with and without diabetes undergoing staged bilateral total knee arthroplasty (TKA). Methods: This was a prospective, non-randomized, comparative study of 38 older adults aged >65 years who were scheduled for staged bilateral TKA at one-week intervals. All older adults fasted from midnight onwards and patients diagnosed with diabetes mellitus (DM group, n = 19) or not (non-DM group, n = 19) were matched for age, sex and body mass index. The primary outcome was residual GV, assessed using ultrasonography. Results: The residual GV (mean (standard deviation)) significantly differed between the DM (75.1 (43.2) mL) and non-DM (35.9 (25.9) mL) groups at the second TKA (p = 0.002) but not at the first TKA (p = 0.096). When comparing the GV at the first and second TKA in each group, a larger GV was observed in the second TKA than in the first surgery in the DM (p < 0.001) and non-DM (p = 0.018) groups. The mean difference in GV amount from the first to second TKA was greater in the DM group (42.5 mL; 95% confidence interval (CI), 26.6–58.5) than in the non-DM group (20.0 mL; 95% CI, 3.8–36.2; p = 0.044). Conclusions: In older patients undergoing staged TKA, the residual GV measured at the first TKA was comparable, irrespective of the presence of diabetes mellitus. However, it increased in older adults with diabetes compared with those without diabetes during the second TKA. The amount of change in residual GV measured from the first to second TKA was greater in diabetic patients than in non-diabetic patients. Hence, in older adults scheduled to undergo bilateral staged TKA, caution is required in preoperative fasting practice, especially at the second stage of surgery in older adults with diabetes. Trial Registration: ClinicalTrials.gov (NCT04815070).