Xanthoria parietina is one of the most widespread and ecologically versatile lichens, yet the diversity of its fungal and algal symbionts and their contribution to its broad ecological niche remain poorly understood. Genetic diversity and phylogenies of both lichen symbionts were inferred from nrITS data. Mycobiont–phycobiont interaction networks were constructed, and ecological niches of associated Trebouxia species were modelled using 19 bioclimatic variables. Phylogenetic analyses revealed high diversity within Xanthoria parietina and clarified the placement of poorly studied species within the genus Xanthoria (e.g., X. monofoliosa and X. aureola s. lat.), and revealed a novel lineage (Xanthoria sp. ‘hydra’). All the photobionts belonged to Trebouxia clade A; comprising nine Trebouxia species-level lineages, including the newineage Trebouxia sp. A56. Trebouxia decolorans (A33) was the most frequent photobiont and exhibited the broadest climatic niche, whereas T. solaris (A35) and T. tabarcae (A48) occupied narrower iches. Species-distribution models predicted widespread suitability for T. decolorans across Europe and coastal-Mediterranean suitability for T. tabarcae. Xanthoria parietina displays remarkable symbiont flexibility in associating with multiple Trebouxia lineages within clade A. This flexibility likely broadens its ecological niche and enhances its ability to thrive across heterogeneous Mediterranean environments. Keywords: ecology | interaction networks | Mediterranean | phylogeny | symbiosis | Trebouxia.