Evidence is accumulating that motor and cognitive development are interrelated. This study investigates associations between motor development in infancy and cognitive function at 4 years of age. This study is part of the Groningen prospective cohort study on the development of children born after assisted reproductive techniques such as in vitro fertilization. The study group consisted of 223 children (119 males, 104 females) born to subfertile couples (median gestational age 39.6wks, range 30-43wks). Motor behaviour was assessed with the Infant Motor Profile (IMP) at 4 months, 10 months, and 18 months. IQ was evaluated at 4 years with the Kaufman Assessment Battery for Children, Second Edition. Latent class growth modelling was used to analyse relations between IMP and IQ scores. Infants with low total IMP scores had an IQ that was 8.9 points lower at 4 years than infants with typical IMP scores (95% confidence interval 3.6-14.1). Also, low scores in the domains of variation and performance were associated with a lower IQ at 4 years of age, by 6.1 points and 13.2 points respectively. Motor development in relatively low-risk infants is associated with cognition at 4 years of age. In particular, low motor variation and performance are associated with a lower IQ at 4 years of age. Motor development in infancy is directly related to cognitive function at 4 years of age. This relationship is most pronounced for infant motor variation and performance. The size of the differences in IQ is clinically relevant.