To identify differences in rumen function as a result of feeding monensin to beef cattle, rumen fluid metagenomics and metabolomics analyses were used to evaluate the functional attributes and metabolites of rumen microbiota in beef steers fed no or 200 mg/d of monensin. Eight rumen-fistulated steers were used in the study for a period of 53 days. Rumen fluid samples were collected on the last day of the experiment. Monensin increased the relative abundance of Selenomonas sp. ND2010, Prevotella dentalis, Hallella seregens, Parabacteroides distasonis, Propionispira raffinosivorans, and Prevotella brevis, but reduced the relative abundance of Robinsoniella sp. KNHs210, Butyrivibrio proteoclasticus, Clostridium botulinum, Clostridium symbiosum, Burkholderia sp. LMG29324, and Clostridium butyricum. Monensin increased the relative abundance of functional genes involved in amino acid metabolism and lipid metabolism. A total of 245 metabolites were identified. Thirty-one metabolites were found to be differentially expressed. Pathway analysis of the differentially expressed metabolites revealed upregulated metabolic pathways associated with metabolism of linoleic acid and some amino acids. These findings confirm that monensin affects rumen fermentation of forage-fed beef cattle by modulating the rumen microbiome, and by reducing amino acid degradation and biohydrogenation of linoleic acid in the rumen.