PURPOSE: The impact of sarcopenia in oncology is increasingly recognized, yet little is known about its clinical implications in breast cancer. This systematic review and meta-analysis estimates the overall prevalence of sarcopenia in breast cancer, quantifies skeletal muscle index (SMI), and comprehensively evaluates sarcopenia's impact on clinical outcomes. METHODS: We systematically searched primary original research published before June 2023 in four databases: the Cochrane Library via Wiley, CINAHL Plus with Full Text, Embase via Elsevier Excerpta Medica, and Medline via Ovid. Standardized mean SMI and 95% confidence interval (CI) were calculated by applying the random-effects model. The methodological quality of the included studies was assessed using the National Institutes of Health quality assessment checklist. RESULTS: The systematic review included 17 studies with a total of 9863 patients; the meta-analysis included 12 of these studies. The mean prevalence of sarcopenia in breast cancer (stages I-III) was 32.5%. The mean SMI assessed by CT was 43.94 cm/m (95% CI 42.87, 45.01; p < .01). Overall, low muscle mass was associated with chemotherapy toxicities, dose reductions, dose delays, or treatment discontinuation. Low muscle mass was generally associated with poor survival, but in some studies, this association was not significant or reversed direction. CONCLUSION: Sarcopenia is not just a state of muscle mass loss, but an influencing factor on therapeutic effects and survival rates in oncology. It is thus necessary to recognize the risk of sarcopenia throughout the trajectory of cancer treatment, identify low muscle mass early, and manage it from a prehabilitation perspective.