Adapazarı (pronounced [adapaˈzaɾɯ]) is a city in northwestern Turkey and the capital of Sakarya Province. The province itself was originally named Adapazarı as well. Adapazarı is a part of the densely populated region of the country, known as the Marmara Region. As of 2010, the city has a population of 560,876 (metropolitan center)

History of Adapazarı dates back to 378 BC. Ancient settlers included Phrygians, Bithynians, Cimmerians, Lydians and Persians, but Adapazarı got its identity from the Romans and Byzantine rulers. One of the most important remains of historical significance is the Justinianus Bridge (Turkish: Beş Köprü) built by Byzantine Emperor Justinian in 533 CE. Historically, it was situated on the old military road from Constantinople (now Istanbul) to the east, connected in Late Antiquity by the important Sangarius Bridge and, since the end of the 19th century, by a branch line with the Anatolian railway.

In 1868, the Adapazarı Municipality was officially founded on 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 Adapazarı and inflicted losses in lives and damage. In the 2000 official census,the population was recorded as 158 474. By official decree, the province was renamed Sakarya after the Sakarya River which runs through it.