Reeri Yakseya or Reeri Yakka is a demon in Sinhalese folklore. It is believed to be the most cruel and powerful demon of all. Although Reeri Yakseya has the body of a human, his head is that of a monkey. His skin is fiery red in colour, and he rides a red bull. It is believed that he can afflict all kinds of illnesses on humans, but hemorrhages and blood diseases are especially attributed to him.

When a man is on his deathbed, Reeri Yakseya is believed to be present at his side. On such occasions, he shows himself as an apparition bearing the appearance of a pygmy. He stands by the dying man holding a cock in one hand and a club in the other, and the corpse of a man in his mouth. This apparition is said to be only one span and six inches in height. When Reeri Yakseya assumes this form, he is known as "Maru Avatar". There are 18 other apparitions or avatars that Reeri Yakseya can assume. These apparitions are known by the following names.

1. Reeri Yakseya (in his original form) 2. Ree Rajja 3. Agu Rajja 4. Pulutajja 5. Reeri Gopalla 6. Reeri Buddia 7. Reeri Watukaya 8. Reeri Billey Dewatawa 9. Reeri Kavisia 10. Reeri Sanniya 11. Reeri Curumberaya 12. Reeri Madana Yakseya 13. Lay Avatar Yakseya 14. Lay Caama Yakseya 15. Marulu Dewatawa 16. Maru Reeri Yakseya 17. Maru Caama Yakseya 18. Maru Avatar Yakseya

A less popular belief is that these are not apparitions of the same demon but 18 individual demons who work together.

Reeri yakseya is believed to have more than a hundred incarnations, each time as a different person. He has been the son of a king named Sanka Pala, a son of the king of Lagal Pura and even a female demon called Ginimuru Yaksani in Hanumanta Desay, among others. Originally, Reeri Yakseya could not be controlled by any of the gods, and all feared him until the god Vishnu managed to bind him with a charmed jungle creeper