Spinal muscular atrophy is a neuromuscular disease characterized by muscle atrophy and progressive muscle weakness due to the degeneration of motor neurons in the anterior horn of the spinal cord. We report a case of an adult patient with spinal muscular atrophy type II and difficulty holding a sitting position. The patient was evaluated before and after Nusinersen treatment and thereafter periodically for up to 3 months for motor and daily living functions. At 3 months post-treatment, the Expanded version of the Hammersmith Functional Motor Scale and the Revised Upper Limb Module, which are motor function assessment tools for evaluating spinal muscular atrophy, showed an increase of 2 points. Evaluation of daily functioning using the Canadian occupational performance measure demonstrated improvements in eating and computer finger manipulation, and these improvements were considered important in daily lives by the patient. This report shows that the Nusinersen treatment improved motor and daily life functions in a patient with spinal muscular atrophy and low motor function. The report also concludes that rehabilitation evaluation for spinal muscular atrophy should include a disease-specific assessment of motor function, combined with an assessment focusing on physical symptoms and daily life functions to capture clinical changes that are responsive to individual patients with spinal muscular atrophy.