There have been many reports about surgical treatments of flexible flatfoot in children and acquired adult flatfoot deformity due to posterior tibial tendon dysfunction common in the 5th and 6th decades. However there has been a controversy for surgical treatment guideline for painful idiopathic flexible flatfoot deformities in young adults. Therefore, we present a 27-year-old female with severe painful idiopathic flexible flatfoot who was treated with medial sliding calcaneal osteotomy and subtalar arthroereisis using $Kalix^{(R)}$ (Newdeal SA, Vienne, France) endoprosthesis and had good clinical outcome with high patient satisfaction at 10 months follow-up postoperatively.