Urban expansion is an important cause of biodiversity loss due to habitat destruction involving the replacement of the natural environment with anthropic infrastructure. However, recent studies suggest that the harmonious growth of cities could allow the persistence of biological diversity within them. Thus, some cities, especially those that contain remnant forests that preceded their expansion, could harbor high levels of biodiversity. These remnant forests, i.e., urban natural forests (UNFs), are refuges for native species, providing suitable habitat conditions for population stability. However, traditional ecological studies have focused on natural and planted forests, so our knowledge of ecological dynamics in UNFs is still limited. Here, we explore the suitability of a temperate UNF for saproxylic beetles. These beetles are deadwood-dependent, highly diverse trophic groups that commonly inhabit natural forest ecosystems. However, saproxylic beetle diversity has been understudied in urban forest ecosystems. We analyzed the species richness, abundance, and seasonal variation of saproxylic beetles, including their most important trophic guilds, associated with urban forests. For this purpose, we monitored standing dead trees monthly between December 2019 and December 2022, using trunk window (flight interception) traps in the UNF Arboretum of Valdivia, Chile. Additionally, we measured qualitative and quantitative habitat attributes to evaluate the preferences of these insects in the UNF. We used generalized linear models as statistical methods to estimate saproxylic beetle habitat preferences. We collected 1273 individuals, belonging to 43 families and 195 saproxylic species/morphospecies. We found that diversity and abundance were strongly influenced by seasonality and the degree of decomposition of deadwood, with increased diversity and abundance in warmer months and highly decomposed trees. Our data show that UNF Arboretum harbor a high diversity of saproxylic species and are therefore suitable habitats and that the factors that regulate these assemblages are homologous to those of natural forests. Finally, our research provides evidence useful for the establishment of conservation and management practices to promote high diversity in UNFs, for example increasing the deadwood volume and quality of the habitat for saproxylic diversity.