The story of Kocaeli, which was called Bithynia in the First Age, begins in the year 712 BC. They came to the Gulf of Izmit from Megara, first settled in Basiskele and founded the city of Astakos, and settled in today's Izmit in 264 BC. This city, the capital of the Kingdom of Bithynia, is called Nicomedia. Nicomedia continued to exist as the Kingdom's capital until it was destroyed.

In 284, Roman Emperor Diocletian invaded Nicomedia and made it the capital of the Roman Empire. Thus, the city becomes the fourth largest city in the world after Rome, Antakya, and Alexandria.

The city's transition to Turkish rule coincides with the Seljuk period in the 11th century. With Iznik becoming the capital of the Anatolian Seljuk State, the importance of the city increases. The city, which was occupied by the Crusader Army for a short time during the Crusades, was definitely under Turkish rule during the reign of Orhan Bey.

Nikomedia, which was annexed to the Ottoman lands by Akça Koca Bey in 1337, took the names first Iznikmid and then Izmid. The commercial and social life of the city, which lived its heyday in the time of Kanuni, was revived with the railway line passing through it in the 19th century. After it became an independent sanjak in 1888, its name was changed to Izmit. Later, it turns into Kocaeli, dedicated to its conqueror, Akça Koca.

The city, which was worn out by the destruction brought by the First World War, was liberated by the Turkish armies on 28 June 1921 from the occupation, first by the British and then by the Greeks. Its proximity to Istanbul and the diversity of transportation opportunities made Kocaeli one of the cities that rapidly industrialized with the Republic. With the SEKA Paper Factory, which was the first paper production facility opened in 1934, the industrialization that has continued until today is still being carried out effectively.