Mahavatar Babaji (IAST: Mahāvatāra Bābājī; lit. 'Great Avatar (Revered) Father') is a legendary immortal yogi and guru, who is said to be living in the Himalayas. He is said to have taught multiple revered historic yogis, including Lahiri Mahasaya (1828–1895), appearing to Lahiri Mahasaya and several of his disciples between 1861 and 1985, as described in various publications and biographies. Babaji's existence was first claimed in the writings of Paramahansa Yogananda, who wrote a chapter of his Autobiography of a Yogi about Babaji and founded Self-Realization Fellowship, a modern yoga movement that Babaji is associated with. According to Paramahansa Yogananda's autobiography, Babaji is said to have resided for at least hundreds of years in the remote Himalayan regions of India, seen in person by only a small number of disciples and others. The cave near Ranikhet where Lahiri Mahasaya said he met Babaji became a tourist attraction and pilgrimage site in India.

The first reported encounter with Mahavatar Babaji (commonly known as Babaji) was in 1861, when Shyāmacharan Lahirī (called "Mahāsaya" by disciples, devotees, and admirers) was posted to Ranikhet in his work as an accountant for the British government. One day while walking in the hills of Dunagiri above Ranikhet, he heard a voice calling his name. Following the voice up the mountain, he met a "tall, divinely radiant sadhu." He was amazed to find that the sadhu knew his name. This sadhu was Mahavatar Babaji.

Mahavatar Babaji told Lahiri Mahasaya that he was his guru from the past, then initiated him into Kriya Yoga and instructed Lahiri Mahasaya to initiate others. Lahiri Mahasaya wanted to remain with Babaji, who told him instead that he must return to the world to teach Kriya Yoga and that "Kriya Yoga sadhana would spread through the people of the world through his (Lahiri's) presence in the world."

Lahirī Mahasaya reported that Mahavatar Babaji did not give his name or background, so Lahiri Mahasaya gave him the title "Mahavatar Babaji." Many sadhus in India are called Babaji, and sometimes even "Babaji Maharaj", which has caused confusion between Mahavatar Babaji and other sadhus with similar names.

Lahirī Mahasaya had many meetings with Babaji, recounted in several books, including Paramahansa Yogananda's Autobiography of a Yogi, Yogiraj Shyama Charan Lahiri Mahasaya (Lahiri's biography), and Purana Purusha: Yogiraj Sri Shama Churn Lahiri.

Several disciples of Lahiri Mahasaya also reported having met Babaji. Through discussion with each other, and the fact that some of these encounters included two or more witnesses, they confirmed that the person they saw was the same sadhu that Lahirī Mahasaya called Mahavatar Babaji.

At the 1894 Kumbha Mela in Allahabad, Yukteswar Giri, a disciple of Lahirī Mahasaya, met Mahavatar Babaji. He was struck by the resemblance between Lahirī Mahasaya and Mahavatar Babaji. Others who met Babaji also commented on the resemblance. It was at this meeting that Mahavatar Babaji instructed Sri Yukteswar to write the book that was to become Kaivalya Darshanam, or The Holy Science. Yukteswar had two more meetings with Mahavatar Babaji, including one in the presence of Lahiri Mahasaya.

Pranabananda Giri, another disciple of Lahirī, also met Mahavatar Babaji in the presence of Lahirī, at Lahirī's home. Pranabananda asked Mahavatar Babaji his age. Mahavatar Babaji responded that he was about 500 years old at that time.