Glashtyn (Manx English: glashtin, glashtan [ˈɡlaʃθən] or glashan; Manx: glashtin or glashtyn [ˈɡlaʃtʲənʲ]) is a legendary creature from Manx folklore.

The glashtin is said to be a goblin that appears out of its aquatic habitat, to come in contact with the island folk; others claim it takes the shape of a colt, or equate it to the water horse known locally as cabbyl-ushtey. Yet another source claims the glashtin was a water-bull (tarroo-ushtey in Manx), half-bovine and half-equine.

Some tales or lore recount that it has pursued after women, ending in the stock motif of escape by cutting loose the skirt-hem, although in one modern version her escape is achieved by a rooster's crowing; in that tale the glashtin pretends to be a handsome man but is betrayed by his horse-ears.

The word glashtin is thought to derive from Celtic glais (Old Irish: glais, glaise, glas), meaning "stream", or sometimes even the sea.

"Glashtin" is the orthography in the Manx language according to Cregeen's dictionary (1835), and this is the spelling adhered to by Joseph Train, A. W. Moore and various other 19th century authorities of Manx folklore.

However the spelling "Glashtyn" is used as heading in John Kelly's Fockleyr dictionary of 1866.

Manx Gaelic glashtin, glashtyn is pronounced /ɡlaʃtʲənʲ/ according to a recent paper on the language.

In the Manx English dialect, "Glashan, glashtan, glashtin" as 'hairy goblin' is the primary (and most detailed) entry given in Moore's posthumous dictionary (1924), completed in collaboration with Morrison and Goodwin.