Purpose: Breast cancer is the most common cancer and leading cause of cancer death in women. Body composition parameters, especially those related to muscle, have become a growing focus of cancer research. In this review, we summarize the literature on breast cancer and muscle parameters as well as combine their outcomes for overall survival (OS), time to tumor progression (TTP), and chemotherapy toxicity in a meta-analysis. Methods: A systematic search of the literature for randomized controlled trials and observational studies was conducted on MEDLINE, Cochrane CENTRAL, and EMBASE through May 1, 2019. Two reviewers independently searched and selected. Meta-analysis was conducted using a random-effects model. The risk of bias was evaluated using the Newcastle–Ottawa quality assessment for cohorts and GRADE summary of findings tool from Cochrane. Results: A total of 754 articles were screened from which 6 articles and one abstract were selected. Using skeletal muscle index (SMI), patients classified as sarcopenic had a 68% greater mortality risk compared to non-sarcopenic patients (HR 1.68 95% CI 1.09–2.59, 5 studies) (p =.02) (i2 = 70%). Low muscle density was not predictive of OS (HR 1.44 95% CI 0.77–2.68, 2 studies) (p =.25) (i2 = 87%). Patients with sarcopenia (56%) had more grade 3–5 toxicity compared to non-sarcopenic (25%) (RR 2.17 95% CI 1.4–3.34, 3 studies) (p =.0005) (i2 = 0%). TTP was nearly 71 days longer in advanced/metastatic patients classified as non-sarcopenic compared to patients with sarcopenia (MD − 70.75 95% CI − 122.32 to − 19.18) (p =.007) (i2 = 0%). Conclusion: Our synthesis of the literature shows that patients with sarcopenia have more severe chemotherapy toxicity as well as shorter OS and TTP, and that low muscle density is prognostic of OS for women with metastatic breast cancer. Our findings suggest that in clinical practice, body composition assessment is valuable as a prognostic parameter in breast cancer.