A 64-year-old woman was treated for an idiopathic complete rupture of the posterior tibial tendon with transfer of the flexor digitorum longus tendon to the navicular. After an initial excellent result, she returned 41 months later after experiencing a sudden pop in the medial retromalleolar area, followed by progressive medial ankle pain. At reoperation, a complete rupture of the flexor digitorum longus tendon was noted at its insertion into the navicular. Reconstruction was performed utilizing a sliding lengthening of the flexor digitorum longus tendon and reattachment to the navicular with a suture anchor. Clinical improvement was noted at the 12-month follow-up evaluation. This case presentation is of clinical interest, as late acute failure of a flexor digitorum longus tendon transfer for posterior tibial tendon rupture has not been reported previously.