Ostrołęka (ostro'łeNka) is a small city in northeastern Poland on the Narew river, about northeast of Warsaw, with a population of 51,012 (2021) and an area of . The town is situated in the Masovian Voivodeship (since 1999), and it is the former capital of the Ostrołęka Voivodeship (1975–1998). Ostrołęka is currently the capital of both Ostrołęka County and Ostrołęka City County.
The city was established in 1373 when it was within the independent Duchy of Masovia. Until the late 1980s, Ostrołęka was a local railroad junction, with four lines stemming from Ostrołęka railway station: eastwards to Łapy and Białystok, southwestwards to Tłuszcz and Warsaw, northwards to Wielbark and Olsztyn, and southwards to Małkinia.
The city's name refers to a sand-mud plain located on the left side of the Narew River which regularly flooded in the springtime throughout the centuries. A small fort was built on an island in the 11th or 12th century, currently located just one kilometer west of the modern city center. The fort was one of few sparsely built fortifications along the Narew River at the time. A village developed surrounding the fort over time, and is associated today as the initial settlement of Ostrołęka. Despite the lack of an exact date for Ostrołęka's founding, it was first mentioned as a town in the Province Act of 1373 by Duke Siemowit III of Masovia, and therefore, the year 1373 has become the most commonly associated date of the acquisition of town rights.
By the beginning of the 15th Century, Ostrołęka grew into an important economic center in the Duchy of Masovia for trade with the neighboring Teutonic Order. Wood, amber, and honey were the most commonly traded items. In 1526, the Duchy was incorporated into the Polish Kingdom. This resulted in an economic boom, most commonly associated to today as Ostrołęka's Golden Age. At this time, Queen Bona Sforza founded a folwark in Pomian – a neighboring settlement at the time, which currently finds itself situated well within the modern city's borders.