Darwin's finch species in the Galapagos Archipelago are an iconic adaptive radiation that offer a natural experiment to test for the various factors that influence gut microbiome composition. The island of Floreana has the longest history of human settlement within the archipelago and offers an opportunity to compare island and habitat effects on Darwin's finch microbiomes. In this study, we compare gut microbiomes in Darwin's finch species on Floreana Island to test for effects of host phylogeny, habitat (lowlands, highlands), and island (Floreana, Santa Cruz). We used 16S rRNA Illumina sequencing of fecal samples to assess the gut microbiome composition of Darwin's finches, complemented by analyses of stable isotope values and foraging data to provide ecological context to the patterns observed. Overall bacterial composition of the gut microbiome demonstrated co-phylogeny with Floreana hosts, recapitulated the effect of habitat and diet, and showed differences across islands. The finch phylogeny uniquely explained more variation in the microbiome than did foraging data. Finally, there were interaction effects for island × habitat, whereby the same Darwin's finch species sampled on two islands differed in microbiome for highland samples (highland finches also had different diets across islands) but not lowland samples (lowland finches across islands had comparable diet). Together, these results corroborate the influence of phylogeny, age, diet, and sampling location on microbiome composition and emphasize the necessity for comprehensive sampling given the multiple factors that influence the gut microbiome in Darwin's finches, and by extension, in animals broadly.