In Irish mythology, Aengus or Óengus is one of the Tuatha Dé Danann and probably originally a god associated with youth, love, summer and poetic inspiration. The son of The Dagda and Boann, Aengus is also known as Macan Óc ("the young boy" or "young son"), and corresponds to the Welsh mythical figure Mabon and the Celtic god Maponos. He plays a central role in five Irish myths.

In Old Irish his name is Óengus or Oíngus [ˈoːi̯ŋɡus], a name attested in Adomnán's Life of St Columba as Oinogus(s)ius. This is believed to come from a Proto-Celtic name meaning "true vigour". The medieval Dindsenchas derives it from "one desire", explaining that Boann gave him the name because her union with the Dagda had been her only desire. In Middle Irish this became Áengus, and in Modern Irish Aonghus [ˈeːnˠəsˠ], [ˈn̪ˠiːsˠ].

He is also known as Óengus Óc/Aengus Óg ("Aengus the young"), Mac ind Óc ("son of the young"), Maccan/Macán, or Mac Óc/Mac Óg ("young son").

According to Celticists John T. Koch and Kenneth H. Jackson, Aengus is also cognate to the Pictish Onuist/Unuist and Old Welsh Unust.

Aengus' parents were The Dagda and Boann, and his foster-father was variously Elcmar or Midir. He was said to have lived at Newgrange by the River Boyne, where he raised Manannán's blonde-haired daughter Curcog as his foster. In the folklore of Scotland, Aengus is the husband of Brigid and son of Beira, the Fairy Queen of Winter. His siblings include Aed, Cermait, Bodb Derg, and Brigid. Aengus is the foster-father and protector of Diarmuid Ua Duibhne of the Fianna. He rescues Diarmuid and Gráinne during their pursuit by the Fianna; after Diarmuid's death, Aengus takes his body back to the Brú na Bóinne where he breathes life into it when he wishes to speak with Diarmuid. According to Death Tales of the Tuatha de Danann, Aengus kills his stepfather Elcmar in retaliation for killing Midir. Aengus has a son named Nemhannach who is mentioned in the Battle of Ventry.

In a late folktale recorded in Scotland, the following physical description is provided:

"Then Angus mounted his white steed and rode eastward...He was clad in raiment of shining gold, and from his shoulders hung his royal robe of crimson which the wind uplifted and spread out in gleaming splendour athwart the sky."

Then a bard composed the following song about Angus: