Hindfoot endoscopy has become popular in last decades for diagnosis and treatment of hindfoot pathologies. Several anatomical landmarks must be needed to control endoscopic anatomy. Flexor hallucis longus (FHL) tendon is an important landmark for the technique. FHL tendon passes from posteromedial aspect of ankle and at talocalcaneal joint level, runs through a fibro-osseous tunnel. Most of FHL tendon pathologies (tenosynovitis or impingement) are seen at the entrance or inside the tunnel. Flexor digitorum accesorius longus (FDAL) has several anatomical variations about its origin and location were defined. It may also be related to FHL tendon but had not usually considered a source of symptoms or functional impairment. We present a case of FHL tendon impingement by FDAL tendon and its diagnosis and treatment with hindfoot endoscopy. It is a very unusual cause of FHL tendon impingement.