Kosala (Sanskrit: , sometimes referred as "Uttara Kosala" ) was one of the sixteen great realms of ancient India. It emerged as a small state during the Late Vedic period and became (along with Magadha) one of the earliest states to transition from a lineage-based society to a monarchy. By the 6th century BCE, it had consolidated into one of the four great powers of ancient northern India, along with Magadha, Vatsa, and Avanti.
Kosala belonged to the Northern Black Polished Ware culture (c. 700–300 BCE) and was culturally distinct from the Painted Grey Ware culture of the neighboring Kuru-Panchala region, following independent development toward urbanisation and the use of iron. The presence of the lineage of Ikshavaku—described as a raja in the Ṛgveda and an ancient hero in the Atharvaveda—to which Rama, Mahavira, and the Buddha are all thought to have belonged—characterized the Kosalan realm.
One of India's two great epics, Ramayana is set in the "Kosala-Videha" realm in which the Kosalan prince Rama marries the Videhan princess Sita.
After a series of wars with neighbouring kingdoms, it was finally defeated and absorbed into the Magadha kingdom in the 5th century BCE. After the collapse of the Maurya Empire and before the expansion of the Kushan Empire, Kosala was ruled by the Deva dynasty, the Datta dynasty, and the Mitra dynasty.
Kosala was bounded by the Gomti River in the west, Sarpika River in the south, Sadanira in the east which separated it from Videha, and the Nepal Hills in the north. It encompassed the territories of the Shakyans, Mallakas, Koliyas, Kālāmas and Moriyas at its peak. It roughly corresponds to modern-day Awadh region in India.
The Kosala region had three major cities, Ayodhya, Saket and Shravasti, and a number of minor towns as Setavya, Ukattha, Dandakappa, Nalakapana and Pankadha. According to the Puranas and the Ramayana epic, Ayodhya was the capital of Kosala during the reign of Ikshvaku and his descendants.