INTRODUCTION: Klippel-Trenaunay syndrome (KTS) is a rare congenital anomaly. Involvement of visceral organs is rarely described, especially in the gastrointestinal system, liver, spleen, mediastinum, and genitourinary system, which typically leads to internal hemorrhage and may present as hematuria or hematochezia. Rectal and bladder bleeding in patients with KTS I are rare conditions. CASE PRESENTATION: We present a rare instance of a 13-year-old female who experienced an unprompted visible hematuria for several days without any additional urinary symptoms. An investigative cystotomy was performed, which unveiled a hemangioma surrounding the opening of the left ureter and a large intramuscular hemangioma on the posterior wall. DISCUSSION: A review of the cases showed that most patients with bladder hemangioma associated with KTS were men with an average age of 16 years (range: 3-37 years). The symptom that we have detected in nearly all cases, including ours, was gross hematuria, except in one case with microscopic hematuria. CONCLUSION: Pediatricians should suspect this syndrome in any child with one of its symptoms, especially since the skin manifestation is distinctive and indicative of this syndrome.