Diarmuid Ua Duibhne (Irish pronunciation: [ˈdʲiəɾˠmˠədʲ uə ˈdˠɪvʲnʲə], lit. 'Diarmuid, grandson of Duibne'), also known as Diarmuid of the Love Spot, is a hero and demigod in the Fenian Cycle of Irish mythology, traditionally thought to be set in the 2nd to 4th century. He is the son of Donn, son of Duibhne of the Fianna, and Cochrann, daughter of Cathaír Mór. Diarmuid becomes the foster son and protégé of Aengus Óg, one of the Tuatha Dé Danann and the god of love, to the extent that a god of love can be said to exist in the corpus. He grows up to be a skilled warrior and a well-liked, valued member of the Fianna who on one occasion saves Fionn and his band by single-handedly slaying over three thousand enemies in battle.

Diarmuid is best known as the lover of Gráinne, the intended wife of Fianna leader Fionn mac Cumhaill in the legend The Pursuit of Diarmuid and Gráinne. Among his sons were Donnchadh, Iollann, Ruchladh and Ioruad.

Diarmuid Ua Duibhne is said to be the founder of the Scottish Clan Campbell. On the Campbell crest is a boar's head, a reference to Diarmuid's geis and death.