The middle spotted woodpecker Dendrocopos medius is a scarce species in northern Spain and is listed as endangered in Catalonia. We show a marked increase over a 24-year period in a population of the middle spotted woodpecker in the Aran Valley in the eastern Pyrenees in Catalonia. The population increased from two pairs in 1990 to 27 pairs in 2010, expanding south and east into two arms of the valley. The estimated breeding density in 2010 was 0.22 pairs/10 ha. Out of 26 surveyed forests, 54% were occupied by the woodpecker. There was no difference in occupancy rates between small, medium-sized or large forest areas. Breeding territories in mixed-deciduous forest were characterised by the presence of large-diameter pedunculate oaks Quercus robur. We conclude that the middle spotted woodpecker is expanding its range in the Pyrenees, probably due to an increasing presence of mature oak forests. The as-yet-small population size of the middle spotted woodpecker in northeastern Spain, combined with its relatively low density, suggests that forest protection measures should remain in place for this regionally vulnerable species.