Bhaktisiddhanta Sarasvati (Bengali: ভক্তিসিদ্ধান্ত ষরস্ৱতী, Bhaktisiddhānta Sarasvatī; February 6, 1874 – January 1, 1937), birth name Bimala Prasad Datta (Bimalā Prasād Datta) — Indian Vaishnava guru and preacher, reformer of Gaudiya Vaishnavism, spiritual teacher of A.C. Bhaktivedanta Swami Prabhupada (1896–1977), the founder of the International Society for Krishna Consciousness (ISKCON).

Bimala Prasad Datta was born in the city of Puri (now in Odisha state) into the family of civil servant Kedarnath Datta (1838–1914), who later gained recognition as an outstanding Gaudiya Vaishnava theologian and preacher. Kedarnath ensured that his son received a good education: both traditional Indian and British. Bimala Prasad's childhood and youth were spent in the then-capital of British India — Calcutta. There he became one of the leaders of educated youth and earned the honorary title "Siddhanta Sarasvati" for his erudition.

In 1901, Bimala received initiation into the Gaudiya Vaishnava tradition from the ascetic Gaurakishora Babaji. He devoted the following years of his life to severe austerities and study of sacred texts. In 1918, he took formal initiation into sannyasa (the renounced order of life), receiving the monastic name "Bhaktisiddhanta Sarasvati Gosvami." That same year, with the goal of propagating Gaudiya Vaishnavism, he founded the missionary organization Gaudiya Math and opened its first center in Calcutta. Soon Gaudiya Math became a dynamically developing "spiritual confederation" consisting of several dozen centers throughout British India. In the early 1930s, Bhaktisiddhanta undertook the first attempt in history to preach Gaudiya Vaishnavism in Europe, which, however, was not successful.

Gaudiya Math was actively engaged in missionary work: publishing and distributing books, newspapers and magazines, organizing public preaching programs. Bhaktisiddhanta was characterized by a passionate and uncompromising preaching style, for which he earned the nickname "lion guru." He consistently opposed the monistic ideas of Advaita Vedanta (which had gained notable popularity in India at the time) and defended the personalist ideas inherent in Gaudiya Vaishnavism and the Krishna-bhakti tradition as a whole. Simultaneously, through word and pen, Bhaktisiddhanta challenged caste prejudices and smarta ideology. He sharply criticized the views of Vaishnava sahajiyas and other Gaudiya Vaishnava currents, which, as he argued, had distorted the ideas of Gaudiya Vaishnavism's founder Chaitanya (1486–1534) and his associates. Bhaktisiddhanta Sarasvati brought the Gaudiya Vaishnava tradition into accordance with the social realities of the 20th century. He considered the publication and distribution of spiritual literature the most effective method of preaching and himself authored numerous translations, commentaries, and philosophical essays.

After Bhaktisiddhanta's death in 1937, his followers proved unable to jointly continue their guru's mission, and Gaudiya Math split into several smaller maths. New life was breathed into Bhaktisiddhanta's movement by his disciple Bhaktivedanta Swami Prabhupada (1896–1977), who founded the International Society for Krishna Consciousness (ISKCON) in New York in 1966 and successfully spread Gaudiya Vaishnavism throughout the world.