Recently, two truncating mutations in the PHF8 (plant homeodomain finger protein 8) gene have been found to cause X‐linked mental retardation associated with cleft lip/cleft palate (CL/P). One of the truncating mutations was found in the original family with Siderius–Hamel CL/P syndrome where only two of the three affected individuals had mental retardation (MR) with CL/P and one individual had mild MR. The second mutation was present in a family with four affected men, three of whom had MR and CL/P, while the fourth individual had mild MR without clefting. Here, we report a novel nonsense mutation (p.K177X) in a male patient who has MR associated with CL/P. The mutation results in a truncated PHF8 protein lacking the Jumonji‐like C terminus domain and five nuclear localization signals. Our finding further supports the hypothesis that the PHF8 protein may play an important role in cognitive function and midline formation.