We report the case of a 40-year-old man presenting with focal headache and a bulge at the right parietal bone, diagnosed as an intradiploic arachnoid cyst. The cyst wall included "meningothelial hyperplasia," which is a rare finding. While over 40 cases of intradiploic arachnoid cysts have been reported to date, meningothelial hyperplasia in an intradiploic arachnoid cyst does not appear to have been reported. We also discuss the pathological findings of arachnoid cysts with meningothelial hyperplasia and mechanisms of enlargement of the arachnoid cyst.