Bláthnat ("Little flower"), sometimes Bláthíne, is a character in early Irish literature, a king's daughter, wife of the warrior Cú Roí and the lover of his rival Cú Chulainn.
Her father is either Mend of Inis Fer Falga (identified as the Isle of Man), Iuchna or Midir, the fairy king of Brí Léith (located in County Westmeath).
Her father's kingdom was invaded by warriors of the Red Branch of Ulster, led by Cú Roí and Cú Chulainn. The raid led to her capture, along with several cattle and a magic cauldron. Despite her being in love with Cú Chulainn, she was chosen by Cú Roí as his personal spoil and marries her, leading to a dispute between the two warriors.
Later, she betrays her husband to his enemies, pouring milk into the River Finglas (Finnglas) as a signal that he was at home. subsequent to this treachery, Cú Roí is slain by Cú Chulainn.
She is later killed by Cú Roí's poet Ferchertne, who avenges his master by throwing both himself and Blathnát from a cliff.
Bláthnat's floral name and the story of her conspiracy have been compared to those of Blodeuwedd in the Fourth Branch of the Mabinogi, Math Uab Mathonwy.