Uddalaka Aruni (fl. c. 8th century BCE), (Devanagari: उद्दालक आरुणि) also referred to as Uddalaka or Aruni or Uddalaka Varuni, was a revered Vedic sage of Hinduism. He is mentioned in many Vedic era Sanskrit texts, and his philosophical teachings are among the centerpiece in the Brihadaranyaka Upanishad and Chandogya Upanishad, two of the oldest Upanishadic scriptures. A famed Vedic teacher, Aruni lived a few centuries before the Buddha, and attracted students from far regions of the Indian subcontinent; some of his students such as Yajnavalkya are also highly revered in the Hindu traditions. Both Aruni and Yajnavalkya are among the most frequently mentioned Upanishadic teachers in Hinduism.

According to Ben-Ami Scharfstein, a professor emeritus of Philosophy at Tel Aviv University, Uddalaka Aruni was one of the first philosophers in recorded history. Debiprasad Chattopadhyaya established Uddalaka Aruni as the first ‘natural scientist’ in the intellectual history of the world. In the Chandogya Upanishad, Aruni asks metaphysical questions concerning the nature of reality and truth, observes constant change, and asks if there is something eternal and unchanging. From these questions, embedded in a dialogue with his son, he presents the concept of Ātman (soul, Self) and Brahman (universal Self).

Uddālaka Āruṇi was a brahmin of the Gautama lineage who was from Kuru-Pañcāla. He was the pupil of Aruṇa (his father) and Patañcala Kāpya. He was the preceptor of Yājñavalkya Vājasaneya, Kahola Kauṣītaki, Proti Kausurubindi, and his own son Śvetaketu Auddālaki. He defeated Prācīnayogya Śauceya and likely Bhadrasena Ājātaśatrava in debate, but was defeated in debate by his own pupil Yājñavalkya.

The name Aruni appears in many of the Principal Upanishads, in numerous verses. For example:

Sage Aruni is revered in the Hindu tradition, and like many of its revered ancient scholars, later-era scholars from the earliest times attributed or named their texts after him. Some of these treatises include:

One very important work of Debiprasad Chattopadhyaya, Science and Society in ancient India, discusses Aruni. In this book he established Uddalaka Aruni as the first "natural scientist" in the intellectual history of mankind. He wrote, "Uddalaka collected observations as far as his historical conditions permitted him and even went on diligently to make experiments to understand nature and man."

According to his theory, there are three elements that make up living things, each of which is a power. The elements are heat, water, and food (meaning plant). He believed the primal creation due to desire emanated heat/light, light having same desire emanated water from itself, then water having the same desire emanated food from itself, which according to him explains why there is plenty of food when it rains. Aruni explained that the existent entered these three elements and when it entered, it caused the elements to contain something of the other two. Fire, for example, not only contains heat but also other two elements.

One of the most well-known teachings of Atman appears in Chandogya Upanishad as the instruction of Uddalaka Aruni to his son Svetaketu. Uddalaka begins his explanation that one can know about the universal of a material substance from a particular object made of that substance. For example, by means of something made of clay, one can know clay; by means of an ornament made of copper, one can know copper. He gives an example for Atman-Brahman doctrine: Just as bees collect nectar from different sources, but when gathered together they form an undifferentiated mass; just as different rivers after flowing into the ocean become one with the ocean, similarly Atman resides individually in every being but merges with the Brahman and becomes one with it. Uddalaka next asks his son to put some salt in a glass of water. Shvetaketu does so. Upon being asked by his father to fetch the salt, Shvetaketu could not find it as it had dissolved in the water. Next, Aruni asks his son to taste the water. Upon being found salty, Aruni preaches Shvetaketu instruction on the Ultimate Reality.