In Irish mythology, Nechtan was the father and/or husband of Boann, eponymous goddess of the River Boyne. Elsewhere her husband is named as Elcmar. He may be Nuada under another name, or his cult may have been replaced by that of Nuada; others maintain that Nechtan may be another name for the Dagda. His inhabited the otherworldly Síd Nechtain, the mythological form of Carbury Hill. In the Dindsenchas Nechtan is described as the husband of Boann and the son of Nuadu.
Only Nechtan and his three cup-bearers were permitted to visit the Tobar Segais, or "Well of Wisdom," into which nine sacred hazel trees dropped their wisdom-bearing nuts. In that well swam the Salmon of Wisdom, which ate the hazelnuts. Eating one of the salmon could in turn imbue a person with knowledge of all things. When Boann visited the well, it overflowed and chased her to the coast, forming the River Boyne.
According to Georges Dumézil the name Nechtan is perhaps cognate with that of the Romano-British god Nodens or the Roman god Neptunus, and the Persian and Vedic gods sharing the name Apam Napat. The name could ultimately be derived from the Proto-Indo-European root *nepot- "descendant, sister's son", or, alternatively, from nebh- "damp, wet". Another etymology suggests that Nechtan is derived from Old-Irish necht "clean, pure and white", with a root -neg "to wash", from IE neigᵘ̯- "to wash"  As such, the name would be closely related mythological beings, who were dwelling near wells and springs: English neck (from Anglosaxon nicor), Swedish Näck, German Nixe and Dutch nikker, meaning "river monster, water spirit, crocodile, hippopotamus", hence Old-Norse nykr "water spirit in the form of a horse".
Nechtan or Nectan became a common Celtic name and a number of historical or legendary figures bear it. Nechtan was a frequent name for Pictish kings. The name MacNaughton derives from "MacNeachdainn", meaning "Son of Nechtan." Nectan of Hartland, said to have lived in the 5th century AD, is the patron saint of Hartland, Devon. St Nectan's Kieve in St Nectan's Glen near Tintagel, Cornwall is said to be named for St. Nectan (5th century AD). Some however argue that St. Nectan never existed as a historical person, but was instead a Christianized form of the god Nechtan.
- Gary R. Varner, Sacred Wells: A Study in the History, Meaning, and Mythology of Holy Wells (Algora Publishing, 2009), p. 26.
- Edel Bhreathnach, entry on "Bóand/Bóinn/Boyne," in Celtic Culture: A Historical Encyclopedia (ABC-Clio, 2006), p. 217.
- The boyhood of Fin mac Cumhal In: T. W. Rolleston (ed.) The High Deeds of Finn and other Bardic Romances of Ancient Ireland, G. G. Harrap & Co., 1910, pp. 106–115.
- Culture, p. 754, citing Dumézil. See also 
- Peter Beresford Ellis, The Druids (Wm. B. Eerdmans Publishing Company, 1995), p. 134.
- Electronic Dictionary of the Irish Language, s.v. necht.
- Koch, entry on "Aedán mac Gabráin," in Celtic Culture, p. 16.
- Varner, op. cit.