The ectopic posterior pituitary (EPP) is a rare condition characterized by the ectopic location of the posterior pituitary lobe associated with varying degrees of stalk anomalies. The arachnoid cysts (AC) are benign lesions of the arachnoid, which account for 1% of all intracranial space-occupying lesions. Sellar/suprasellar ACs account for approximately 1% of all ACs. This is the first case of coexistence EPP with sellar/suprasellar AC. A 67-year-old woman presented with 6 months history of fatigue. Her medical history was positive for irregular menstruation. Her endocrine examinations indicated low free thyroxine level with low TSH level, low oestradiol with low gonadotrophin level, slightly elevated prolactin level. Her Insulin-like growth factor-1 was below the normal levels. Dynamic contrast hypophysis MRI revealed a sellar cystic lesion with a dimension of 18 × 14 × 14 mm, extending from the suprasellar cistern, traversing the diaphragma sellae and reaching the level of the floor of the 3rd ventricle, consistent with sellar/suprasellar AC. There was no wall enhancement. The optic chiasm was compressed. The precontrast T1-weighted magnetic resonance images did not demonstrate the characteristic bright spot of posterior pituitary within the sella, which was higher in position, in the region of the median eminence. The pituitary stalk was not present. Although speculative, we have a hypothesis to explain how the EPP and sellar/- suprasellar AC coexist in this patient. Due to the absence of stalk, CSF may enter the sella tursica from the central aperture of the diaphragma sellae through which normally the stalk passes.