Chełmno land is a historical region, located in central-northern Poland.
The first historical account of Chełmno and Chełmno Land dates back to 1065 when Bolesław II of Poland granted a tax privilege to an abbey in a nearby Mogilno. The document lists Chełmno ("Culmine") along with other towns which then belonged to the province of Masovia. The area, being closest to the Polans, came to be populated by the Lechitic Kuyavians and tribes from Greater Poland. The Masovians were led by Masos, who left the Polish duke Boleslaw I and sought refuge with the Prussians. When this area was subdued by the rulers of the Polans Chełmno became a local centre of administration (kasztelania). Chełmno Land was Christianised in the 11th century.
According to the will of Duke Bolesław III Wrymouth, Chełmno Land, after his death in 1137 became a part of the Duchy of Masovia governed by his son Bolesław IV the Curly and his descendants during the feudal fragmentation of Poland. However the Prussians as well as the Pomeranians many times managed to regain their freedom from subjugation by the expanding Polish dukes.
By the 13th century the territory was subject to raids by Prussians, who sacked Chełmno, the province's main town, in 1216. In 1220 Conrad I of Masovia, with the participation of the other princes of Poland, led a partial reconquest of the province, but the project of establishing a Polish defense of the province failed due to conflicts between the princes. He brought the crusading Knights of Dobrzyń to Masovia, where they built a castle at Dobrzyń in 1224 as a base for attacks against the Prussians. As a result the territory was again sacked and devastated by Prussian raids, which led to depopulation of the province.
Being involved in dynastic struggles elsewhere and too weak to deal with the Prussians alone, Conrad needed to safeguard and establish borders against the heathen Old Prussians, because his territory of Masovia was also in danger after the Prussians besieged Płock. Conrad awarded the already devastated Chełmno Land to the Teutonic Knights, giving them Nieszawa at first. He also brought in German settlers to Płock.
In 1226 Duke Conrad I of Masovia enlisted the aid of the Teutonic Order to protect Masovia and help convert the Prussians to Christianity. In return, the knights were to keep Chełmno Land as a fief. The land constituted the base of the Monastic State of the Teutonic Knights, and its later conquest of East Prussia.
The Teutonic Order obtained an Imperial bull from Emperor Frederick II before entering Prussia. In 1243 the papal legate William of Modena divided Prussia into four dioceses under the archbishop of Riga, one of which was Chełmno Land.
After the Thirteen Years' War between the Prussian cities and the Teutonic Knights ended with the Second Peace of Thorn (1466), Chełmno Land returned to the Polish crown as a part of Royal Prussia (see Chełmno Voivodship).
In 1772 as a result of the First Partition of Poland, Chełmno Land (with the exception of Toruń) was seized by the Kingdom of Prussia. Between 1807 and 1815 Chełmno Land was a part of the Duchy of Warsaw. In 1815 it become part of the Grand Duchy of Posen, but in 1817 was incorporated into West Prussia.
Following the Treaty of Versailles, Chełmno Land was returned to Poland in January 1920. It was occupied in the Invasion of Poland in September 1939 by Nazi Germany and annexed in October. In January 1945 it was captured by Red Army and returned to Poland.