Objective Cerebral palsy (CP) is the most prevalent motor disability during childhood, characterized by a range of movement and posture disorders that result in activity limitations. In children with CP, abnormal walking patterns are observed due to spasticity and contractures in the lower limb muscles. Also, an excessive flexion gait pattern of the knee presents in 66% of spastic diplegic CP children aged from 5 to 21 years. Accordingly, reducing excessive knee flexion during walking may decrease secondary compensations and enhance walking performance and quality of life. This study aims to design and evaluate a novel dynamic orthosis along with its impact on functional parameters of walking and occupation performance in a child with spastic diplegia CP. Materials & Methods This research was a fundamental and applied study of the design and fabrication. The study plan is a single subject, interaction design, conducted in two stages and implemented as A-B-BC-A. After the initial orthosis design, a second and final iteration was developed to address the limitations observed in the preliminary design. The study participant included an 8-year-old boy with spastic diplegic CP, classified as level II (according to the gross motor function classification system expanded and revised), exhibiting a crouch gait pattern. Functional variables, such as speed, balance in walking, spasticity, and occupation performance, were assessed in the study phases. Results The advantages and disadvantages of the final design were identified. Changes were made regarding manufacturing in less time, the ability to adjust the circumferential measures, ease of wearing and removing, and issues related to toileting. In comparing the baseline with intervention phases one and two regarding balance, the intervention demonstrated effectiveness (percentage of non-overlapping data [PND]=70%). According to PND values, functional speed in the second intervention phase showed significant results (PND=75%). The hamstring muscle spasticity graph analysis during intervention phases exhibited a reduction of 1 or 2 units. The occupational performance score and satisfaction with the child’s performance increased significantly after the intervention phases. Conclusion According to the mechanism of dynamic movement orthoses, as long as they are worn during the day, they are effective in modulating the tonicity of spastic muscles by exerting pressure on hypertonic muscles. This modulation improves balance, movement speed, and overall task performance. Consequently, these findings suggest that dynamic orthotic interventions may enhance functional outcomes and occupational performance, and satisfaction in children with spastic diplegic CP.