Shantanu (Sanskrit: शांतनु, शान्तनु, IAST: Śāṃtanu, Śāntanu) is a character in the ancient Hindu epic Mahabharata. He was the King of Kuru Kingdom with his capital at Hastinapura. He was a descendant of the Bharata race, a forebear of the lineage of the Chandravamsha, the father of Bhishma and the great-grandfather of the Pandavas and the Kauravas.

Shantanu was the youngest son of King Pratipa of Hastinapura and had been born during the king's later years. His eldest brother, Devapi, had leprosy, and had given up his rightful claim before becoming a hermit. The middle son, Bahlika (or Vahlika), abandoned his birth kingdom, and had started living with his maternal uncle in Balkh, subsequently inheriting his kingdom. Shantanu, thus, ascended the throne of Hastinapura.

The meaning of the name can be explained by nirukti available in Adi Parva, through which Sri Nityānanda Miśra elaborates its meaning as "the one who amplifies sukha (happiness) for others". Monier-Williams translates śaṁ-tanu as "wholesome for the body".

The Sambhava Parva of Mahabharata says that the old men of his kingdom who were touched by this monarch not only felt an indescribable sensation of pleasure but also became restored to youth. Therefore, this monarch was called Santanu.

In his previous birth, there was a powerful king of the Ikshvaku dynasty named Mahabhisha. He possessed many virtuous qualities, and after performing a thousand Ashvamedha Yagnas and a hundred Rajasuya Yagnas (to qualify as emperor), he had attained heaven after his death. Once, he got an opportunity to visit the court of Brahma where all the Devas and Ganga were also present. While the celestials were worshipping Brahma, a wind blew and displaced Ganga's clothes revealing her body. Everybody present there lowered their gaze except Mahabhisha who kept gazing at her. Upon seeing this, Brahma lost his temper and cursed him to be born a mortal. Ganga who also relished the impious act was cursed to be born as human and come back only after breaking Mahabhisha's heart. Mahabhisha then requested Brahma that he, Mahabhisha, be born as the son of Kuru king Pratipa and his wish was granted by Brahma.

When the Pratipa was once meditating, Ganga took the form of a beautiful woman, approached the king and sat on his right thigh. When he asked her what she desired, Ganga requested him to become her husband. Pratipa however refused since he had taken a vow not to lust for anybody, and also that she had sat on his right thigh and according to traditions a man's right thigh was for his daughter or daughter-in-law while the left thigh was for his wife. He then proposed that she marry his son, to which she agreed.

A child was born to Pratipa and his wife Sunanda in their old age. He was named Shantanu because when he was born his father had controlled his passions by ascetic penances. Pratipa then installed Shantanu as king of Hastinapura and retired into the woods to perform penances. Bahlika, who was elder than Shantanu, also gave his assent to Shantanu for becoming the king of Hastinapura.

Shantanu saw a beautiful woman on the banks of the river Ganga and asked her to marry him. She agreed but with one condition: that Shantanu would never question any of her actions. They married and later she gave birth to a son. But she drowned the child. Shantanu could not ask her the reason, because of his promise, lest she would leave him. Six sons were born to the couple over the years and drowned by Ganga.