The glaistig /ˈɡlæʃtɪɡ/ is a ghost from Scottish mythology, a type of fuath. It is also known as maighdean uaine (Green Maiden), and may appear as a woman of beauty or monstrous mien, as a half-woman and half-goat similar to a faun or satyr, or in the shape of a goat. The lower goat half of her hybrid form is usually disguised by a long, flowing green robe or dress, and the woman often appears grey with long yellow hair. A sighting of the glaistig is rare, but the loud cries and wails would often be heard.

The glaistig is an ambivalent ghost that appears in legend as both a malign and benign creature. Some stories have her luring men to her lair via either song or dance, where she would then drink their blood. Other such tales have her casting stones in the path of travellers or throwing them off course.

In some benign incarnations, the glaistig is a type of tutelary spirit and protector of cattle and herders. One of the legends in Scotland, in the town of Ach-na-Creige there was such a spirit protecting the cattle herds. The townsfolk, in gratitude, poured milk from the cows into a hollowed-out stone for her to drink. According to the same legend, her protection was revoked after one local youth poured boiling milk into the stone, burning her. She has also been described in some folklore as watching over children while their mothers milked the cows and fathers watched over the herds.

In some Scottish tales, the cows are replaced with deer, of which the glaistig is fiercely protective. Also known as the Scottish goddess of the hunt, the glaistig was both help and hindrance to hunters, going so far as to hide her herds if the hunters made the mistake of killing a doe instead of a stag. Occasionally, hunters would be granted permission to shoot if they provided an offering, such as the hind of the deer to the herder. Another name of this instance is the Maiden of Callart.