The mammalian gut harbors a very complex and diverse microbiota that play an important role in the intestinal homeostasis and host health. Exposure to radiation results in dysbiosis of the gut microbiota leading to detrimental pathophysiological changes to the host. To alleviate the effects of irradiation, several candidate countermeasures are under investigation. BIO 300 is a promising candidate under advanced development. The aim of this study was to investigate the effects of BIO 300 on the gut microbiome and metabolome of mice exposed to ⁶⁰Co gamma-radiation. The gut microbiota and metabolome of control and drug treated mice exposed to irradiation was characterized by bacterial 16S rRNA amplicon sequencing and untargeted metabolomics. We found that irradiation, altered the ratio of Bacteroidetes to Firmicutes as well as significantly decreased the relative abundance of Lactobacillus, initially, both in BIO 300-treated and control mice; however, the ratio returned to near normal levels in BIO 300-treated mice by day 14 post-irradiation. Concomitantly, we also observed corrective shifts in metabolic pathways that were perturbed after irradiation. Overall, the data presented show that radiation exposure led to a bloom of opportunistic pathogens and depletion of beneficial commensals leading to an inflammatory metabolic phenotype while majority of the drug treated mice showed alleviation of this condition primarily by restoration of normal gut microbiota. These results indicate that the radioprotective effects of BIO 300 at least in part may involve correction of the host-microbiome metabolic axis.