Hirayama disease is a rare, self-limiting lower motor neuron disorder predominantly affecting young males in Asia. It has an insidious onset with distal upper limb weakness and atrophy. The underlying pathogenesis is caused by dynamic compression of the lower cervical spinal cord during neck flexion, resulting in ischemic injury to the anterior horn cells. A 15-year-old Indonesian male presented with 6-months of progressive right upper limb weakness and muscle wasting without sensory deficits or spasticity. He had noticed symptoms when he had gradual neck soreness and difficulty writing. Examination showed right thenar eminence atrophy and weakness in the C6-T1 myotomes. Nerve conduction studies showed no evidence of peripheral neuropathy or brachial plexopathy. Electromyography showed motor neurogenic changes with ongoing denervation and fasciculations in the right upper limb, with possible anterior horn cell involvement. Initial cervical MRI in the neutral position appeared normal. However, repeat dynamic MRI cervical spine demonstrated anterior displacement of the posterior dural sac and dilatation of the posterior epidural venous plexus from C3–C6 with neck flexion, confirming the diagnosis of Hirayama disease. The patient was managed conservatively with physiotherapy and a hard cervical collar for 6 weeks to limit neck flexion before transitioning to soft collar. At 8 months’ follow-up, symptoms continued to be stable with no further progression. Hirayama disease is often underdiagnosed due to normal findings on neutral MRI cervical spine. Flexion imaging is essential for detecting the hallmark anterior dural displacement and posterior epidural venous engorgement. Early recognition and conservative management with a cervical collar can prevent further neurological deterioration by limiting neck flexion. Although rare, Hirayama disease should be considered in adolescents presenting with unilateral distal upper limb weakness. Dynamic MRI cervical spine is crucial for diagnosis, and early use of a cervical collar can halt disease progression and preserve neurological function.