["Journal of Intellectual Disability Research, Volume 70, Issue 6, Page 599-607, June 2026. ", "\nABSTRACT\n\nObjective\nTo explore the association between 24‐h movement behaviours and fundamental motor skills in children with intellectual disabilities using compositional data analyses and to investigate the ‘dose‐effect’ characteristics of the reallocation between 24‐h movement behaviours and fundamental motor skills.\n\n\nMethods\nA cross‐sectional study was conducted among 306 children with intellectual disabilities aged 6–10 years from 12 special education schools in Beijing and Jinan between 10 September 2023 and 27 March 2024. The ActiGraph GT3X+ accelerometer was used to estimate the amount of time spent in 24‐h movement behaviours. The Test of Gross Motor Development‐2 was applied to assess fundamental motor skills. The compositional isotemporal substitution was utilized to analyse the relationship between 24‐h movement behaviours and fundamental motor skills.\n\n\nResults\n(1) After controlling the gender, age and intellectual disability level, MVPA of children with intellectual disabilities was positively associated with their FMS total score, locomotor skills and object control skills (βFMS = 7.70, βlocomotor = 3.68, βobject control = 4.01, all p < 0.01). Additionally, SB was negatively correlated with their FMS total score, locomotor skills and object control skills (βFMS = −5.40, βlocomotor = −2.00, βobject control = −3.39, all p < 0.01). (2) According to the ‘dose–response’ curves, the mutual substitution of MVPA and other movement behaviours had an asymmetric effect on FMS, while the mutual substitution of LPA and SB had a symmetrical effect on FMS among children with intellectual disabilities. Furthermore, it was demonstrated that replacing SB with MVPA had the best‐improving effect on the FMS of children with intellectual disabilities.\n\n\nConclusion\nSpecial education school administrators, teachers, parents and guardians should consider 24‐h movement behaviours as a whole and pay attention to their impact on children with intellectual disabilities. In the process of promoting FMS in children with intellectual disabilities, ensuring adequate sleep and trying to reallocate time from SB to MVPA and LPA may be effective methods.\n\n"]