Adapazari is a municipality and the capital district of Sakarya Province, Turkey. Its area is 324 km², and its population 281,489 (2022). It covers the central and northern part of the agglomeration of Adapazari and the adjacent countryside.

The history of Adapazari dates back to 378 BC, when it was called Agrilion. Ancient settlers included Phrygians, Bithynians, Cimmerians, Lydians, Greeks, and Persians, but Adapazari got its identity from the ancient Hellenistic, Roman, and Greek Byzantine rulers. After Alexander the Great's conquests, the Persians were forced out of the region.

One of the most important remains of historical significance is the Sangarius Bridge built by Byzantine Emperor Justinian in 533 AD. Historically, it was situated on the old military road from Constantinople (now Istanbul) to the east, connected, since the end of the 19th century, by a branch line with the Anatolian railway. It was founded in 1400 by 400 Armenian immigrant families who escaped from Timur's oppression in Sivas and was named "Donigasen" after the name of their leaders. Today's inhabitants are Manavs. The native Muslims are Armenians and Greeks who gradually converted to Islam, and it should not be ignored that Turk households settled in the region. Adapazari's known name in Turkish at that time was Adacik and Ada. The source of the information that the region was conquered by Orhan Gazi and opened to settlement is also unknown.

In 1868, the Adapazari Municipality was officially founded in the town called Der Seadet. A sugar factory and agricultural sugar factory attracted many migrants and accelerated the industrialization of the city. A law passed on 17 June 1954 in the Grand National Assembly saw the city separated from Kocaeli Province and made the center of its own province. The 1999 Marmara earthquake affected Adapazari and inflicted losses in lives and material damage. In 2008 the district Adapazari was created from part of the former central district of Adapazari Province, along with the districts Arifiye, Erenler and Serdivan. At the same time, the province was renamed Sakarya after the Sakarya River which runs through it.