Herein, we present a study of an interesting sample of fossils of the giant amphicyonid Megamphicyon giganteus (Schinz, 1825) from the Spanish middle Miocene (MN6) site of Carpetana (Madrid city), obtained during public works for the Madrid underground in 2008. Although the dentition of this species is known from other sites, the postcranial bones are very poorly known, and the new material provides new data on the locomotor adaptations of this spectacular predator, and allows an estimation of its body mass, which would be around 600 kg. Surprisingly, despite this giant size, the morphology of both femur and tibia shows that this animal exhibited higher cursorial capacity than large extant carnivorans, such as bears, which together with a high degree of pronation/supination movement of the forearm, reveals adaptations of M. giganteus to occupy the niche of active top predator in middle Miocene ecosystems.