This study evaluated the effects of feeding dried distillers grains with solubles (DDGS) or reduced-fat DDGS (RFDG) on the rumen bacterial community composition. Four ruminally cannulated Holstein dairy cows were used in a 4 × 4 Latin square over 28-d experimental periods. Treatments (DM basis) were (1) CONT, a diet with no distillers grains; (2) DG, inclusion of 20% DDGS; (3) rfDG, inclusion of 20% RFDG; and (4) MIX, inclusion of 10% DDGS and 10% RFDG. The DDGS and RFDG were included by replacing corn grain and soybean meal. At the end of each period, the rumen bacterial community was evaluated by sequencing the 16S rRNA gene. The phylum Bacteroidetes decreased and Firmicutes tended to increase when distillers grains and solubles were fed. No treatment effect was observed on the predominant bacterial families Prevotellaceae (P = 0.51), Lachnospiraceae (P = 0.67), Veillonellaceae (P = 0.69), Spirochaetaceae (P = 0.45) and Paraprevotellaceae (P = 0.87) with averages across treatments of 29.37 ± 1.430%, 7.23 ± 0.367%, 5.89 ± 0.963%, 3.04 ± 0.218%, and 2.21 ± 0.151%, respectively. However, the abundance of the genus Ruminococcus tended (P = 0.06) to increase with estimates of 1.72 and 2.22 ± 0.230% for CONT and treatments containing distillers grains and solubles, respectively. In addition, the abundance of the genus Bulleidia increased (P < 0.01) with estimates of 0.35 and 0.46 ± 0.027% for CONT and treatments containing distillers grains and solubles, respectively. The archaeal genera Methanobrevibacter (P = 0.86) and Methanosphaera (P = 0.85) were not affected by treatment and averaged 1.64 ± 0.234% and 0.11 ± 0.023%, respectively, across treatments. Overall, this study demonstrates that DDGS or RFDG can be used to replace corn and soybean meal in dairy rations; however, in doing so, the abundance of certain bacterial taxa within the rumen are shifted.