Malignant psoas syndrome (MPS) is a rare, painful condition caused by malignant infiltration of the psoas muscle and adjacent nerves, causing fixed flexion of the leg over the hip. Unfortunately, it is often underdiagnosed. We presented the case of an 8-year-old girl with a clinical history of lumbar embryonal rhabdomyosarcoma and high-grade sarcoma presenting as MPS. To the best of our knowledge, this is the first pediatric case report described in Mexico.