In the times of the first Piasts, a defensive fortress in the fortification belt of the so-called Łańcut Gate was located in the area of today's Leżajsk.
Leżajsk was first mentioned in historical documents in 1346. It was officially chartered as a town in 1397 under King Władysław II Jagiełło. Leżajsk was a royal town, governed by a district governor from 1435. The town was repeatedly destroyed during Tatar invasions in the 15th and 16th century. In 1524 the town was chartered again in a new place, more convenient for its defence. Since that moment, Leżajsk became an important handicraft and trade centre. Crops and other goods were floated to Gdańsk up the San and Vistula rivers.
Fires and wars in the 17th and 18th century blocked the growth of Leżajsk. From 1655 to 1656, the invading Swedes destroyed the town during the Swedish Deluge. Upon the First Partition of Poland in 1772, Leżajsk came under the Austrian rule. The Austrians took the status of a district town away from Leżajsk, moving the capital to Łańcut. In 1809, the troops of the Duchy of Warsaw commanded by Prince Józef Poniatowski entered the town.
By the end of the 19th century, upon the construction of the railway line Przeworsk–Rozwadów, the town experienced economic recovery. During World War I Leżajsk was severely destroyed during numerous battles over the San River. During WWII, in September 1939, the town was occupied by the German troops, while in July 1944 it was occupied again by the Soviet army.
Currently, Leżajsk is a local administrative, economic, cultural, and educational centre for neighbouring towns, as well as an important industrial centre in the privince.
