Kosambi (Pali) or Kaushambi (Sanskrit) was an ancient city in India, characterized by its importance as a trading center along the Ganges Plain and its status as the capital of the Vatsa Kingdom, one of the sixteen mahajanapadas.

It was located on the Yamuna River about 56 kilometres (35 mi) southwest of its confluence with the Ganges at Prayaga (modern Prayagraj), which made it a powerful center for trade and beneficial for the Vatsa Kingdom.

Kosambi was a very prosperous city by the time of Gautama Buddha, where a large number of wealthy merchants resided.

The present location of the Kosambi pillar inside the ruins of the fort attests to the existence of Mauryan military presence in the region.

"[7] In the post-Mauryan period a tribal society at Kosambi (modern Prayagraj district) made cast copper coinage with and without punchmarks.

More than three thousand stone sculptures have been recovered from Kausambi and its neighbouring ancient sites –7 Mainhai, Bhita, Mankunwar, and Deoria.

The excavations of the archaeological site of Kosambi was done by G. R. Sharma of Allahabad University in 1949 and again in 1951–1956 after it was authorized by Sir Mortimer Wheeler in March 1948.

Another explanation is[14] that large and stately neem trees or Kosammarukkhā grew in great numbers in and around the city.

Near Kosambī, by the river, was Udayana/Udena's park, the Udakavana, where Ananda and Pindola Bharadvaja preached to the women of Udena's palace on two occasions.

The Buddha spent his ninth rainy season at Kosambī, and it was on his way there on this occasion that he made a detour to Kammāssadamma and was offered in marriage Māgandiyā, daughter of the Brahmin Māgandiya.

When the matter was reported to the Buddha, he admonished the partisans of both sides and urged them to give up their differences, but they paid no heed, and even blows were exchanged.

In disgust, the Buddha left Kosambī and, journeying through Bālakalonakāragāma and the Pācīnavamsadaya, retired alone to keep retreat in the Pārileyyaka forest.

In the meantime the monks of both parties repented, partly owing to the pressure exerted by their lay followers in Kosambī, and, coming to the Buddha at Sāvatthi, they asked his pardon and settled their dispute.

The last phase dated to 1st - 2nd century CE featured an extensive structure which was divided into three blocks and enclosed two galleries.

The palace had a vast network of underground chambers and the superstructure and the galleries were made on the principle of true arch.

The superstructure of central and eastern block was examined to have formed part of a dome that adorned the building.

The entire galleries and superstructure were found collapsed under 5 cm thick layer of ash which indicates destruction of the palace through conflagration.