Kāśī (Pali: Kāsī) was an ancient kingdom of India whose existence is attested during the Iron Age. The inhabitants of the Kāśī were named the Kāsikas in Pāli and the Kāśeyas and Kāśikas in Sanskrit.

The Kāśī kingdom covered an area of 300 leagues. The northern border of Kasi which separated it from Kosala was the Sarpikā or Syandikā river, and the river Son formed its southern and eastern boundaries, separating it from Magadha in the east.

The capital of Kāśī was the city of Vārāṇasī, which was also named Ketumatī, Surundhana, Sudassana, Brahmavaddhana, Pupphavatī, Ramma, and Molinī.

The Kāśikas were first mentioned in the Paippalāda recension of the Atharvaveda.

The ruling clan of Kāśī appears to have been member of the Bhārata clan, and at one point Kāśī was ruled by one Dhṛtarāṣṭra (in Sanskrit) or Dhataraṭṭha (in Pāli) whom the Mahāgovinda Suttanta calls a "Bharata prince." This Dhṛtarāṣṭra was defeated in battle by another Bharata king, named Śatānīka Sātrājita, after which the Kāśikas stopped kindling the sacred fire until the time of the Śatapatha Brāhmaṇa.

However, the monarchs of Kāśī do not appear to have all belonged to the same dynasty, and the Jātakas frequently mention the extinction of dynasties of the Kāśika kings or the deposition of Kāśika princes and their replacement with members from other families regarded as being more competent, with some kings of Kāśī having Māgadhī or Vaideha origins and bearing the epithet of Brahmadatta: although Brahmadatta was a dynastic name, the Brahmadattas were not from the same dynasty, with the elected Brahmadatta of the Darīmuka Jātaka having been a Māgadhī prince, the king of Kāśī in the Dummedha Jātaka and his son being both named Brahmadatta, the Kāśika king Udaya being called "Brahmadatta" in the Gaṅgamāla Jātaka, while the Brahmadattas of the Matiposaka and Sambula Jatakas were Vaidehas.

The Kāśikas were closely connected to the Kausalyas and the Vaidehas, and Jala Jātūkarṇya was the purohita of these three kingdoms during the reign of the Kāśika king Ajātaśatru, who was himself a contemporary of the famous Vaideha king Janaka and of Uddālaka Āruṇi's son Śvetaketu. Ajātaśatru appears in the Upaniṣads as engaging in philosophical discussions with Gārgya Bālāki, and the Kauśītaki Upaniṣad depicts Ajātaśatru as being jealous of Janaka's fame as a patron of learning. The Uddālaka Jātaka calls this Ajātaśatru of Kāsī "Brahmadatta," implying that he was himself was a Brahmadatta.

The Śatapatha Brāhmaṇa mentions a king of Kāsī named Bhadrasena Ajātaśatrava, who was likely the son and successor of Ajātaśatru, and had been bewitched by Uddālaka Āruṇi.