Understanding how biodiversity is distributed across geographical and environmental gradients is a main goal of diversity sciences. However, since ecosystem processes are linked to variation in functional traits of the biota, examining functional beta diversity is particularly important. Our objective was to analyze the taxonomic and functional beta diversity patterns of saproxylic beetle assemblages in evergreen Quercus forest of Spain. We tested whether environmental or geographical distance had a greater influence on taxonomic and functional beta diversity, and if both measures of beta diversity were affected by the same environmental variables. We used 45 flight interception traps distributed in three protected areas over a 12-mo period to sample saproxylic beetles. We measured 13 environmental variables around each trap and the geographical distance between traps. For functional composition, we used 12 functional traits from four functional groups (morphological, phenological, trophic, and a surrogate of physiological). Our results showed that environmental differences between areas influenced the taxonomic and functional beta diversity components (replacement and loss/gain) but in different ways. While replacement components (higher for taxonomic composition) increased with environmental distance, the loss or gain components (higher for functional composition) remained constant, indicating that species replacement mostly involved functionally redundant species. Besides, environmental variables influencing both taxonomic and functional composition were strongly dependent on each area. In conclusion, in well-preserved Mediterranean forests, environmental filtering determines the taxonomic and functional composition of saproxylic beetle assemblages, by favoring species replacement but filtering species traits.