Background It is widely believed that increasing parental involvement can improve children's educational outcomes although we lack good evidence for such claims. This study evaluated the effectiveness of a parent‐delivered early language enrichment programme. Methods We conducted a randomised controlled trial (RCT) with 208 preschool children and their parents living in socially diverse areas in the United Kingdom. Families were allocated to an oral language programme (N = 103) or an active control programme targeting motor skills (N = 105). Parents delivered the programmes to their child at home in daily 20‐min sessions over 30 weeks of teaching. Results Children receiving the language programme made significantly larger gains in language (d = .21) and narrative skills (d = .36) than children receiving the motor skills programme at immediate posttest. Effects on language were maintained 6 months later (d = .34), and at this point, the language group also scored higher on tests of early literacy (d values=.35 and .42). There was no evidence that the movement programme improved motor skills. Conclusions This study provides evidence for the effectiveness of a parent‐delivered language enrichment programme. Further large‐scale evaluations of the programme are needed to confirm and extend these findings.