Purpose The Communication Function Classification System (CFCS) and Viking Speech Scale (VSS) are useful systems for describing the broad communication function and speech intelligibility, respectively, of children with cerebral palsy (CP). The aims of this study were to determine the reliability and validity of the Korean version of the CFCS and also to investigate the association between the CFCS and the VSS and other functional classifications for children with CP. Materials and methods Participants were 50 children with CP (33 males, 17 females; mean age 7.2 years, range 4–16 years) recruited from a rehabilitation hospital. We analysed the interrater and intrarater reliabilities of the Korean version of the CFCS and VSS between parents, a physiatrist, and a speech–language pathologist (SLP). The social function domain of the Paediatric Evaluation of Disability Inventory was assessed to examine the concurrent validity of the CFCS and VSS. Results The intrarater reliabilities of the CFCS and VSS were excellent in a physiatrist (ƙ = 0.92, ƙ = 0.94, respectively) and an SLP (ƙ = 0.98, ƙ = 0.98) and very good in parents (ƙ = 0.87, ƙ = 0.89). The interrater reliability of the CFCS and VSS was very good between the physiatrist and SLP (ƙ = 0.87, ƙ = 0.89) and good between parents and the SLP (ƙ = 0.63, ƙ = 0.78) and between parents and the physiatrist (ƙ = 0.61, ƙ = 0.76). The CFCS and VSS were strongly related with the social function domain of Paediatric Evaluation of Disability Inventory. In addition, we found very strong associations between the VSS and CFCS. Conclusions The Korean version of the CFCS is a valid and reliable tool to classify communication ability and is strongly associated with the VSS, a reliable tool to classify speech intelligibility.