This study aimed to examine the effect of pool walking on renal function in pregnant women. Fifteen pregnant women (mean gestational age, 37.8 weeks) walked in a pool (depth 1.3 m) for 1 h. A few days later, they walked on a street for 1 h. Within each activity, the starting and ending levels of plasma renin activity were measured. The total urine volume, creatinine clearance, and change in plasma renin activity levels between each activity were compared by Wilcoxon rank-sum test. The renin- angiotensin-aldosterone level was suppressed during pool walking: the mean starting and ending values of plasma renin activity and serum aldosterone were 6.8 vs. 5.5 ng/mL/h (p = 0.002) and 654 vs. 473 pg/mL (p = 0.01), respectively. The decreases in plasma renin activity and serum aldosterone levels were more evident in pool walking than in land walking (plasma renin activity, −1.27 vs. 0.81 ng/mL/h, p = 0.002; serum aldosterone, −180.9 vs. 3.1 ng/mL/h, p = 0.03), resulting in higher total urine volume (299 vs. 80 mL, p < 0.001) and creatinine clearance (161.4 vs. 123.4 mL/min, p = 0.03) in pool walking. Pool walking may improve renal function in pregnant women partly through the suppressed renin-angiotensin-aldosterone system.