Sri Yukteswar Giri, born Priya Nath Karar in Serampore, West Bengal in 1855, was a well-known Indian monk, yogi, educator, author, and astronomer. He was a disciple of Lahiri Mahasaya and a member of the Giri branch of the Swami order. Sri Yukteswar was a Kriya yogi and an expert in Vedic astrology, the Bhagavad Gita, and the Upanishads. He was the guru of Paramahansa Yogananda and Swami Satyananda Giri, among others.

Described as having a gentle and pleasing presence, Sri Yukteswar was a progressive-minded figure in 19th-century Serampore society. He held religious festivals throughout the year, created a spiritual study organization, and established educational syllabi. He also re-analyzed the Vedic astrological yugas. Sri Yukteswar was respected by his Kriya yoga students and regularly invited individuals from all social backgrounds to his ashrams to discuss a range of topics.

Sri Yukteswar had two ashrams, one in Serampore and another in Puri, Odisha, where he alternated his residence throughout the year while training disciples. He was known for his sharp mind and insightful knowledge, and his teachings continue to inspire and guide many spiritual seekers today.

As a guru, he was nonetheless known for his candid insight, stern nature and strict disciplinary training methods, as noted by his disciple Yogananda in his autobiography.[6] The rigorous nature of his training eventually prepared his disciples, such as Satyananda and Yogananda himself, for their own intense social work in India and America, respectively. In accordance with the high ideals and "penetrating insight" with which he lived, Sri Yukteswar was considered by Yogananda as a Jnanavatar, or "Incarnation of Wisdom;"[5] Evans-Wentz felt him "worthy of the veneration which his followers spontaneously accorded to him...Content to remain afar from the multitude, he gave himself unreservedly and in tranquility to that ideal life which Paramhansa Yogananda, his disciple, has now described for the ages."[5]

Sri Yukteswar was born as Priya Nath Karar in Serampore, India, to Kshetranath and Kadambini Karar. Despite losing his father at a young age, he showed great promise as a student and attended Srirampur Christian Missionary College, where he developed an interest in the Bible. Later, he also briefly attended Calcutta Medical College. After leaving college, Priya Nath married and had a daughter, but tragically, his wife passed away a few years later.

In 1884, Sri Yukteswar met Lahiri Mahasaya, who became his Guru and initiated him into the path of Kriya Yoga. He spent a considerable amount of time with his Guru and frequently visited Lahiri Mahasaya in Benares. At the Kumbha Mela in Allahabad in 1894, Sri Yukteswar met Mahavatar Babaji, the Guru of Lahiri Mahasaya, who requested him to write a book comparing Hindu scriptures and the Christian bible. Sri Yukteswar completed the book, named The Holy Science or Kaivalya Darsanam, that same year. At the meeting, Mahavatar Babaji also bestowed upon Sri Yukteswar the title of 'Swami'. He later went on to become a respected Guru in his own right, training disciples in Kriya Yoga and inspiring many with his teachings on the unity of all religions.

Sri Yukteswar established two ashrams, one in his family home in Serampore called "Priyadham" and the other in the town of Puri called "Karar Ashram". He taught students from these ashrams and founded an organization called "Sadhu Sabha". Additionally, he was interested in education and developed a syllabus for schools that included physics, physiology, geography, astronomy, and astrology. He even wrote a book to teach Bengalis basic English and Hindi. Sri Yukteswar was skilled in Jyotiṣa (Indian astrology) and prescribed gemstones and bangles based on astrological beliefs. He was also knowledgeable in astronomy and science, which is evident in his Yuga theory presented in his book, The Holy Science. Furthermore, he was an advocate for women's education, which was rare in Bengal during his time.

Despite having only a few long-term disciples, Sri Yukteswar had a significant impact on the spiritual world, especially through his most famous disciple, Paramahansa Yogananda. Yogananda spread the teachings of Kriya Yoga worldwide through his Self-Realization Fellowship/Yogoda Satsanga Society of India. However, Sri Yukteswar's strict training methods led to his limited number of disciples. He believed that the guru's role was to assist in the blossoming of knowledge and to provide techniques for sadhana, which would enable the disciple to become a sage and elevate their soul. Sri Yukteswar was a gentle, pleasing presence, and he was held in the highest esteem by all who knew him. He passed away on 9 March 1936 at Karar Ashram, Puri, India.

Sri Yukteswar authored "The Holy Science" in 1894 to illustrate the fundamental unity of all religions, the truth inculcated by different faiths, and the one goal admitted by all scriptures. The book contained innovative ideas such as the earth's advancement from Kali Yuga to Dvapara Yuga and the sun taking a 'star for its dual' and revolving around it in 24,000 years, which explained the precession of the equinox. Sri Yukteswar's work on the Sun's binary companion and its role in Indian history has drawn attention from David Frawley, who has written about it in several of his books. The Binary Research Institute conducted research on this theory and produced a documentary on it called "The Great Year," which is narrated by James Earl Jones. Additionally, there is an iPhone application available for calculating Sri Yukteswar's calculations.