Hormones influence and coordinate a suite of traits in response to environmental or social change. Though there is strong support for the phenotypic-integration role of steroid hormones, only recently have researchers started investigating the relationship between hormones and the microbiome, another host trait known to influence individual fitness. Previous work has demonstrated that gut microbiota can influence hormonal regulation and host behavior, mainly focusing on glucocorticoid hormones and the stress response. Even though androgens, like testosterone, regulate changes in physiology and behavior to help match organisms to their social environment, their impact on the gut microbiome is unclear. Using red-backed fairywrens (Malurus melanocephalus), a bird in which the effect of testosterone on breeding, behavior and plumage ornamentation is well documented, we tested the hypothesis that testosterone also influences the gut microbiome, possibly through differences in foraging behavior. We collected gut samples from unornamented testosterone-implanted males, unornamented sham-implanted males (controls), as well as females and ornamented males. We measured microbiome and diet diversity (as a proxy for foraging) using DNA metabarcoding. We found that implanted males have significantly lower microbial alpha diversity in the intestine than control males, though there was no difference in diet diversity. Our study highlights the complex ways in which testosterone may influence gut microbiota in relation to different phenotypes, and how testosterone may have direct effects on the microbiome rather than indirect effects through a change in foraging or diet, and warrants further work to disentangle how these relationships influence host fitness.