Wilms tumor (WT), or nephroblastoma, is a rare pediatric kidney cancer that poses significant clinical challenges. Despite advancements in treatment, WT survivors face the risk of secondary malignant neoplasms (SMNs). This report discusses an 11-year-old girl who developed a secondary juvenile granulosa cell tumor of the ovary eight years after being treated for stage III WT. Initial treatment included nephrectomy, chemotherapy, and radiation. The secondary tumor, identified through imaging and surgery, highlights the importance of extended follow-up. This case underscores the need for vigilance in monitoring WT survivors for late-onset complications, even beyond the typical five-year surveillance period.