The microbiome is defined as the collective genomes of the microbes (composed of bacteria, bacteriophage, fungi, protozoa, and viruses) that colonize the human body, and alterations have been associated with a number of disease states. Changes in gut commensals can influence the neurologic system via the brain-gut axis, and systemic insults such as trauma or traumatic brain injury (TBI) may alter the gut microbiome. The objective of this study was to evaluate the gut microbiome in a preclinical TBI cortical impact model. Male rats underwent craniotomy and randomized to a sham group (n = 4), or a moderate TBI (n = 10) using a pneumatic impactor. MRI and behavioral assessments were performed pre-TBI and on days 2, 7, and 14 days thereafter. Microbiome composition was determined with 16s rRNA sequencing from fecal sample DNA pre-TBI and 2 hrs, 1, 3, and 7 days afterward. Alpha- and β-bacterial diversity, as well as organizational taxonomic units (OTUs), were determined. Significant changes in the gut microbiome were evident as early as 2 h after TBI as compared with pre-injured samples and sham rats. While there were varying trends among the phylogenetic families across time, some changes persisted through 7 days in the absence of therapeutic intervention. While large structural lesions and behavioral deficits were apparent post-TBI, there were modest but significant decreases in α-diversity. Moreover, both changes in representative phyla and α-diversity measures were significantly correlated with MRI-determined lesion volume. These results suggest that changes in the microbiome may represent a novel biomarker to stage TBI severity and predict functional outcome.