The poorer recognition performance for inverted as compared to upright faces is one of the most well-known and robust behavioral effects observed in the field of face perception. Here we investigated whether extensive training at individualizing a large set of inverted faces in adulthood could significantly reduce this inversion effect for novel faces. This issue is important because inverted faces are as complex as upright faces but they are not visually experienced during development. Moreover, inverted faces violate the biological constraints, present at birth, for preferential looking (i.e., a larger number of elements in the top part than the bottom part of the stimulus). Eight adult observers were trained for 2 weeks (16 hr) to individualize 30 inverted face identities presented under different depth-rotated views. Following training, all participants showed a significant reduction of their inversion effect for novel face identities presented in a challenging four-alternatives delayed matching task. This reduction of the face inversion effect was observed in comparison to the magnitude of the same observers' effect before training, and to the magnitude of the face inversion effect of a group of untrained participants. These observations indicate that extensive training in adulthood can lead to a significant reduction of the inversion effect that generalizes to novel faces, suggesting a larger degree of flexibility of the adult face processing system than previously thought.