This study investigated the effects of sensory-perceptual-motor and cognitive functions on handwriting skill in primary-school children with left-hemiplegic cerebral palsy, compared with that of their healthy peers. The study included 26 children aged 8-12 years with left hemiplegic cerebral palsy and 32 typically developing children of similar age with dominant right hand. The Minnesota Handwriting Assessment was used to evaluate handwriting skill. The Bruininks-Oseretsky Test of Motor Proficiency was used to assess motor performance. Cognitive function was assessed by the Lowenstein Occupational Therapy Cognitive Assessment. The Ayres Southern California Sensory Integration Tests were used to assess visual perception, kinaesthesia and graphesthesia. Statistically significant differences were found between the two groups in sensory-perceptual-motor and cognitive function and handwriting skill (p < 0.05). There were also significant correlations between handwriting parameters and upper-extremity speed and dexterity, proprioception, bilateral coordination, visual and spatial perception and, visual-motor organisation in children with cerebral palsy (p < 0.05). The results showed that left-hemiplegic children with cerebral palsy whose right sides were dominant were significantly less competent at handwriting than their right-dominant, healthy peers. It was found that the impairment in proprioception seen in the non-hemiplegic side in children with cerebral palsy, and also the impairment in bilateral coordination, speed and dexterity of the upper extremities, visual and spatial perception, visual-motor organization, and tactile-sensory impairments negatively affected their handwriting skills. In the treatment approaches for children with hemiplegic cerebral palsy, comprehensive sensory-perceptual-motor assessments that involve both extremities must be performed in detail at the earliest possible stage, in order to minimize the existing problems with early-treatment policies. Developing the sensory-perceptual-motor and cognitive function of hemiplegic children would thus be possible, and they would be able to develop handwriting skill as a tool for their academic lives as healthy peers.