A 63-year-old man with type 2 diabetes mellitus developed deep aching and numbness in the right hip and lower extremity with rapid body weight loss. Neurological examination revealed weakness of the right hamstrings, tibialis anterior, and peroneus longus muscles with diminished ankle tendon reflex. We diagnosed him with diabetic lumbosacral radicuoloplexus neuropathy (DLRPN) based on neurological, radiological, and neurophysiological findings. Magnetic resonance imaging (MRI) of skeletal muscles showed high intensity signals on T2-weighted images in bilateral hamstrings, adductor magnus and right tensor fasciae latae, and lower leg extensor muscles. The MRI findings suggested muscle edema caused by acute denervation. DLRPN, or diabetic amyotrophy, is known to be caused by ischemic axonal degeneration. Our patient showed good functional recovery, and abnormal MRI signals in the involved muscles mostly disappeared in parallel to the clinical course. Distribution of the denervated muscles suggested that our patient had either patchy lesions in the lumbosacaral plexus or mononeuropathy multiplex in the nerve branches. The current study highlights the potential of skeletal muscle MRI for clinical evaluation of DLRPN.