Aruni or Uddalaka or Uddalaka Aruni (fl. c. 7th century BCE) was a philosopher of Vedic India, whose teachings are recorded in the Brihadaranyaka Upanishad and Chandogya Upanishad. One of the first philosophers in recorded history, he taught that everything in the universe (including all living things) is made of three elements: heat (or light), water, and food (or plants), and furthermore that all of these were manifestations of the universal Self or Ātman. It has been suggested that his teachings contain the seeds of Indian atomism, because of his belief that "particles too small to be seen mass together into the substances and objects of experience".
Aruni hailed from the country of Panchala and was known as Aruni of Panchala. His son was Svetaketu.