Atrogin-1 and muscle RING finger 1 (MuRF1) are ubiquitin ligases specifically expressed during skeletal muscle atrophy and mediate muscle protein degradation. In contrast, PGC-1α (peroxisome proliferator-activated receptor γ coactivator 1α), which is a master regulator of mitochondrial biosynthesis, protects skeletal muscle from atrophy. Pyrimidine nucleoside 5′-monophosphates, such as cytidine 5′-monophosphate (5′-CMP) and uridine 5′-monophosphate (5′-UMP), induce PGC-1α expression and promote myotube formation in mouse C2C12?cells. In this study, we determined the effect of 5′-CMP and 5′-UMP on muscular atrophy in C2C12 myotube cells. 5′-UMP decreased Atrogin-1 and MuRF1 mRNA levels that were upregulated by dexamethasone treatment. 5′-CMP and 5′-UMP ameliorated dexamethasone-mediated atrophy in C2C12 myotubes. Furthermore, the combination of 5′-CMP and 5′-UMP further alleviated dexamethasone-mediated atrophy. In addition, cytidine and uridine, the precursors of 5′-CMP and 5′-UMP, markedly ameliorated dexamethasone-mediated atrophy. Considering nucleotide metabolism and absorption, the active metabolites underlying the observed effects of 5′-CMP and 5′-UMP appear to be cytidine and uridine. Our results indicate that 5′-CMP alleviates muscle atrophy by activating PGC-1α and differentiation, and 5′-UMP alleviates muscle atrophy by suppressing the activation of the myolytic system, whereas the combined use of both enhances the muscle atrophy inhibitory effect. 5′-CMP and 5′-UMP may be an effective and safe treatment for muscular atrophy.