PPP2R1A mutations have recently been described in 3/42 (7%) of clear cell carcinomas of the ovary. PPP2R1A encodes the α-isoform of the scaffolding subunit of the serine/threonine protein phosphatase 2A (PP2A) holoenzyme. This putative tumour suppressor complex is involved in growth and survival pathways. Through targeted sequencing of PPP2R1A, we identified somatic missense mutations in 40.8% (20/49) of high-grade serous endometrial tumours, and 5.0% (3/60) of endometrial endometrioid carcinomas. Mutations were also identified in ovarian tumours at lower frequencies: 12.2% (5/41) of endometrioid and 4.1% (2/49) of clear cell carcinomas. No mutations were found in 50 high-grade and 12 low-grade serous carcinomas. Amino acid residues affected by these mutations are highly conserved across species and are involved in direct interactions with regulatory B-subunits of the PP2A holoenzyme. PPP2R1A mutations in endometrial high-grade serous carcinomas are a frequent and potentially targetable feature of this disease. The finding of frequent PPP2R1A mutations in high-grade serous carcinoma of the endometrium but not in high-grade serous carcinoma of the ovary provides clear genetic evidence that these are distinct diseases.