The Iberian lynx (Lynx pardinus) reintroduction in the Guadiana valley (Portugal) began in 2015. By 2019 more than 100 lynxes were in the wild, dispersed over 300 km² . The arrival of a new top predator (the lynx had been extinct from this region for decades) may have affected red fox populations (Vulpes vulpes) as a result of intraguild competition. The aim of this study was to evaluate the effects of the lynx reintroduction on the spatial, temporal and spatial-temporal dimensions of local fox abundance and behaviour. Camera trapping results revealed a clear decrease in fox abundance. Also, co-occurrence occupancy models indicated a higher probability of fox occupancy in areas where lynxes were absent, as well as a greater fox detectability in places where lynxes were absent, rather than in places where lynxes were present. In the temporal dimension, there were no significant effects on fox behaviour. However, the spatial-temporal dimension revealed a competitive exclusion since foxes avoided areas that had recently been used by lynxes.