Shesha (Sanskrit: शेष, romanised: Śeṣa, lit. 'Remainder'), also known by his epithets Sheshanaga (Sanskrit: शेषनाग, romanised: Śeṣanāga, lit. 'The snake Shesha') and Adishesha (Sanskrit: आदिशेष, romanised: Ādiśeṣa, lit. 'First Shesha'), is a serpentine demigod (naga) and king of the serpents (Nagaraja), as well as a primordial being of creation in Hinduism. In the Puranas, Shesha is said to hold all the planets of the universe on his hoods and to constantly sing the glories of Vishnu from all his mouths. He is sometimes referred to as Ananta (Sanskrit: अनन्त, lit. 'Without end') or Ananta Shesha (Sanskrit: अनन्त शेष, romanised: Ananta Śeṣa, lit. 'Endless Śeṣa/Remainder').

The Narayana form of Vishnu is often depicted as resting on Shesha, accompanied by his consort Lakshmi. Shesha is considered as one of the two mounts of Vishnu alongside Garuda. He is said to have descended upon Earth in the following human forms or incarnations: Lakshmana, brother of Vishnu's incarnation Rama during the Treta Yuga, and according to some traditions, as Balarama, brother of Vishnu's incarnation Krishna during the Dvapara Yuga. According to the Mahabharata (Adi Parva), his father was Kashyapa and his mother Kadru, though in other accounts, he is usually a primordial being created by Vishnu.

His name means "he who remains", from the Sanskrit root śiṣ, because even as the world is destroyed at the end of each kalpa, Shesha remains as he is.

Shesha is generally depicted with a massive form that floats coiled through space, or upon the Ocean of Milk, to form the bed upon which Vishnu lies. Sometimes, he is depicted as a five-headed or seven-headed or a ten-headed serpent; but more commonly as one thousand-headed, or five thousand-headed, or even as many as a one million-headed serpent; sometimes with each head wearing an ornate crown.

According to the Mahabharata, Shesha was born to the sage Kashyapa and his wife Kadru. Kadru gave birth to a thousand snakes, of which Shesha was the eldest. After Shesha, Vasuki, Iravati and Takshaka were born, in that order. A lot of Shesha's brothers were cruel and were bent upon inflicting harm on others. They were even unkind to Garuda, who was Kashyapa's son through Vinata, sister of Kadru. (Kadru and Vinata were daughters of Daksha).

Shesha, disgusted by the cruel acts of his brothers, left his mother and kin, and took on austere penances. He lived on air and meditated in places including Gandhamadhana, Badrikashrama, Gokarna, Pushkara, and the Himalayas. His penances were so severe that his flesh, skin, and muscles dried up and merged with his frame. Brahma, convinced of Shesha's will, asked Shesha to request a boon. Shesha asked that he be able to keep his mind under control so that he could continue to perform ascetic penances. Brahma gladly accepted the request. Brahma then asked a favour of Shesha: to go beneath the precarious earth and stabilise it. Shesha agreed and went to the netherworld and stabilised Bhumi with his hood. He is believed to support her even today, thus making Patala his perennial residence.

Shesha is usually depicted as floating in the ocean of the changing world, forming the bed of a primordial form of Vishnu, i.e. Narayana, Vāsudeva or in later Puranic Vaishnavism, Mahavishnu.

In the Bhagavata Purana, Shesha is named Sankarshana, the tamasic energy of Narayana himself, and is said to live deep within the inner layers of Patala, where there are many serpents with gems on their heads and where he reigns as its ruler. He is said to have existed before the creation of the universe. When the universe is towards its end, he creates the 11 Rudras from the serpents to destroy the universe for a new one to be created.