Severe malnutrition accompanied by sarcopenia and cachexia, is strongly associated with the surgical and oncological outcomes in cancer patients. The aim of the present study was to clarify the clinical significance of sarcopenia and its correlation with sarcopenia-associated miRNA in colorectal cancer (CRC). A total of 167 CRC patients were enrolled in the present study. We evaluated psoas muscle mass index (PMI) and intramuscular adipose tissue content (IMAC). The expression of miR-21 in CRC tissues and preoperative serum was evaluated using quantitative PCR. Despite the lack of significant correlation between IMAC and disease-correlated factors, decreased PMI was significantly associated with well-established clinicopathological factors for disease progression. Decreased PMI was an independent prognostic factor for both overall survival and disease-free survival and was an independent risk factor for various types of metastasis. In contrast to the expression of tissue miR-21, the expression of serum miR-21 was significantly increased in CRC patients with low PMI. Furthermore, postoperative PMI was drastically improved compared with preoperative PMI in CRC patients with potentially curative resections. In conclusion, skeletal muscle mass may be a prognostic and predictive biomarker for distant metastasis in CRC patients and quantification of serum miR-21 expression could help clinicians make decisions regarding nutrition intervention strategies in CRC patients.