Background: Several studies suggested the prognostic importance of sarcopenia in survival or treatment complications in cancer. The concept of cancer prehabilitation before surgery is gaining wide recognition. The aim of our study was to assess the impact of sarcopenia on chemotherapy toxicities and survival in patients with localized muscle invasive bladder cancer (MIBC). Methods: Between January 2012 and December 2017, patients who underwent neoadjuvant platinum-based chemotherapy (NAC) followed by radical cystectomy (RC) for cT2-T4 N0 M0 MIBC were retrospectively selected, from a single institution cohort. Using CT scan: before NAC, after NAC and after cystectomy, sarcopenia was defined according to the Lumbar Skeletal Muscle Index (SMI) in L3 assessed by two observers blinded to patients’ status. Results: 82 patients were selected, 62 men (75.6%) and 20 women (24.4%), with a median age of 64.5 years (31 to 80). 35 were considered severe sarcopenic (SMI < 35cm²/m² for women and SMI < 50 cm²/m² for men) before NAC. The characteristics between severe and non-severe sarcopenic patients were not significantly different except for weight (71.7 vs 81.1kg, p = 0.006) and BMI (24.0 vs 28.4kg/m², p = 8.0x10^-6). Sarcopenia pre-chemotherapy was significantly associated with nausea (p = 0.003), hypokalemia (p = 0.023) and with platinum dose-intensity during NAC (p = 0.007) but was not significantly associated with overall survival (OS) or disease-free survival (DFS). Sarcopenia post-cystectomy was correlated with the OS (HR = 0.94, CI95% [0.89-0.99], p = 0.01), but not DFS. In multivariate analysis, factors associated with OS were post cystectomy sarcopenia (p = 0.038), pN status (p = 0.001) and glomerular filtration rate (p = 0.045). Conclusions: For localized MIBC, sarcopenic status before platinum based NAC could predict some chemotherapy toxicities. After radical cystectomy, sarcopenia is an independent prognostic factor for OS. Improve patients care by prehabilitation during NAC and before surgery, using physical, nutritional and psycho-social supports, could decrease chemotherapy toxicities and improve survival particularly for sarcopenic patients.