In Irish mythology, Nechtan was the father and/or husband of Boann. He may be Nuada under another name, or his cult may have been replaced by that of Nuada. His inhabited the otherworldly Síd Nechtain, the mythological form of Cadbury Hill.
Only Nechtan and his three cup-bearers were permitted to visit the well of Segais, into which nine sacred hazel trees dropped their wisdom-bearing nuts. When Boann visited the well, it overflowed and chased her to the coast, forming the river Boyne.
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. It may also be cognate to the Swedish mythological being Näcken, who dwells near wells and springs.
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", the son of Nechtan.