Objectives: To study the effectiveness of application of indoor socks with medial arch support in decreasing pain in patients with posterior tibial tendon dysfunction (PTTD). Study design: Prospective, double-blinded, randomized controlled trial Setting: Foot Clinic, Department of Rehabilitation Medicine, Siriraj Hospital Subjects: Patients with PTTD and foot pain at the out-patient clinic, during August 2017 – August 2018 Methods: Forty–two patients were randomized into two groups, the study group (using socks with medial arch support, Rehband®) and the control group (using socks with flat foam). All were advised to wear them while walking and standing indoors for 2 weeks. Complication and duration of using the socks were recorded in a logbook. Foot pain was assessed with numeric rating scale (NRS) before and after 2 weeks of using them. Pre-post differences in pain and success rates of pain reduction (at least two points) were compared between the two groups. Results: After using the socks for 2 weeks, there were statistically significant decreases in foot pain in both groups (the study group, p < 0.001 and the control group, p = 0.004). However, the pre-post intervention differences in foot pain between two groups was not statistically significant (p = 0.07). The success rates of pain reduction were statistically significant difference between the two groups (the study group 80.9% and the control group 42.9%, p = 0.025). No serious complication was reported. Conclusion: In patients with posterior tibial tendon dysfunction, using indoor socks with medial arch support showed no statistically significant difference in decreasing foot pain when comparing with the control group using socks with flat foam. However, the former had higher success rate in pain reduction than the latter.