Overgrowth syndromes comprise a group of heterogeneous disorders characterised by excessive growth parameters, often in association with intellectual disability. To identify new causes of human overgrowth, we have been undertaking trio-based exome sequencing studies in overgrowth patients and their unaffected parents. Prioritisation of functionally relevant genes with multiple unique de novo mutations revealed four mutations in PP2A regulatory subunit B family genes PPP2R5B, PPP2R5C and PPP2R5D. This observation in three related genes in 111 individuals with a similar phenotype is greatly in excess of the expected number, as determined from gene-specific de novo mutation rates (P=1.43x10^–10). Analysis of exome sequencing data from a follow-up series of overgrowth probands identified a further pathogenic mutation, bringing the total number of affected individuals to five. Heterozygotes shared similar phenotypic features including increased height, increased head circumference and intellectual disability. The mutations clustered within a region of nine amino acid residues in the aligned protein sequences (P=1.6x10^–5). We mapped the mutations onto the crystal structure of the PP2A holoenzyme complex to predict their molecular and functional consequences. These studies suggest that the mutations may affect substrate binding, thus perturbing the ability of PP2A to dephosphorylate particular protein substrates. PP2A is a major negative regulator of AKT. Thus, our data further expands the list of genes encoding components of the PI3K/AKT signalling cascade that are disrupted in human overgrowth conditions.