Helena Kantakouzene (Greek: Ἑλένη Καντακουζηνή, romanized: Eleni Kantakuzini; 1333 – 10 December 1396) was the Empress consort of John V Palaiologos of the Byzantine Empire. She served as Regent during the absence of her son Manuel II in 1393.^[1]

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She was a daughter of John VI Kantakouzenos and Irene Asanina; Donald Nicol believes she was the youngest of their three daughters.^[2] She was a sister of Matthew Kantakouzenos and Manuel Kantakouzenos. Her sisters Maria and Theodora were the respective wives of Nikephoros II Orsini and Orhan.

John V and John VI were rival emperors in a civil war fought from 1341 to 1347. The two sides at last reached an agreement. According to its terms John VI would be recognized as senior co-emperor with John V as his junior. The peace was sealed with the marriage of Helena to John V.

Helena and John V had at least eleven children— five sons and at least six daughters.^[6] These known children include:

• Andronikos IV Palaiologos (2 April 1348 – 28 June 1385);
• Irene Palaiologina (c. 1349 – after 1362), who married her first cousin Halil Bey. Her husband was a son of Orhan and Helena's sister Theodora Kantakouzene. The couple had two sons, Gündüz Bey and Ömer Bey.
• Manuel II Palaiologos (27 June 1350 – 21 July 1425);
• Theodore I Palaiologos, Lord of Morea (c. 1355 – 24 June 1407);
• Michael Palaiologos (d. 1376/1377), who claimed the throne of the Empire of Trebizond from Alexios III;
• Maria Palaiologina, married Murad I;
• One daughter betrothed to Peter II of Cyprus, who may not be Irene or Maria;
• An unnamed daughter reported to have entered a monastery in 1373, who may be different women from the ones listed above.
• An unnamed daughter who married Bayezid I, son of Murad I;
• An unnamed daughter who married Yakub Çelebi, son of Murad I.

A fictionalised form of her character features prominently in the novel Adora by Bertrice Small, published 1980.^{[citation needed]}