Background: Sarcopenia (SP) is described as a prognostic factor in adult and pediatric cancer patients. However, there is no data regarding Wilms Tumor (WT). Our aim was to study the association between sarcopenia and oncological outcomes in WT. Methods: A retrospective study of patients diagnosed with WT at our institution between 2010-2022 was performed. SP at diagnosis was assessed by measuring the psoas muscle area (PMA) at the L4-L5 level on CT/MRI, and was defined as z-score values <-2. Demographics, complications and outcomes were analyzed. Results: Forty-eight patients (50% male) were included, with a mean age of 44.91±31.12 months. Twelve patients (25%) had SP at diagnosis vs. 36 (75%) who did not. Forty-one patients (85.4%) underwent total nephrectomy and 7 (14.6%) nephron-sparing surgery (NSS). No statistical differences were found in demographics, risk group or treatment between the SP and non-sarcopenic (NSP) groups. SP was associated with a higher rate of postsurgical complications (33.3% for the SP-group vs. 5.6% for the NSP-group; p=0.023) and with a higher rate of relapse (33.3% vs. 13.8% respectively; p=0.09). With a median follow-up of 57.75 (1.87-150.8) months, event-free survival (EFS) was lower for the SP-group (84.20±17.45 vs. 135.40±8.65 months respectively; p=0,08). One patient in the SP-group died. The 5-years overall survival (OS) was 89% for the SP-group vs. 100% for the NSP-group. Conclusions: Among our patients, SP can be considered as a risk factor for complications in patients with WT and could be associated with poor outcomes, increasing the risk of relapse and decreasing EFS.