Objective To investigate the effect of aqua therapy resistance exercise on arm volume, pain, and shoulder range of movements in post-mastectomy lymphedema.Methods This was a single-blind randomized controlled trial. Fifty eligible breast cancer survivors (median, 10 years after surgery) with lymphedema (median, 21% inter-limb difference) were assigned randomly to group A (n=25) or control group B (n=25). The study group underwent 60 minutes of aqua therapy exercise comprising of warm-up for 10 minutes, 40 minutes of strengthening exercises, and 10 minutes of cooling down, three times a week for 8 weeks. The control group underwent 60 minutes of land-based exercise three times a week for 8 weeks. Arm volume calculated by measuring the arm circumference, shoulder flexion, and abduction range of motion (ROM), and pain using a visual analog scale (VAS) were assessed at baseline and after 8 weeks of treatment.Results There was a statistically significant difference in limb volume, shoulder flexion and abduction ROM, and VAS scores in favor of the study group (p<0.001) after 8 weeks of intervention. The mean±standard deviation for limb volume, shoulder flexion, abduction, and pain score were 2,108.71±200.97 mL, 169.68°±4.54°, 150.44°±3.92°, and 3.16±1.1 in the study group and 2,256.41±186.94 mL, 147.36°±5.32°, 131.32°±4.38°, and 5.68±0.94 in the control group, respectively.Conclusion Adding aqua therapy resistance exercise to routine physical therapy might be more effective in decreasing the limb volume and pain intensity and improving ROM of the shoulder in postmastectomy lymphedema.