The Northern Crusades or Baltic Crusades were military campaigns undertaken by Catholic Christian military orders and kingdoms in an effort to Christianize the pagan Baltic, Finnic, and West Slavic peoples around the southern and eastern shores of the Baltic Sea. In some cases, such as with the Wendish Crusade, the campaign was partly motivated to control the rich resources found in the lands.

The most notable of these campaigns were the Livonian and Prussian Crusades. Some of these wars were explicitly regarded, during the Middle Ages, as crusades; for example, the military venture against the Estonians—and the "other pagans in those parts"—authorized by Pope Alexander III's 1171 crusade bull, Non parum animus noster. However, others—such as the (possibly mythical) 12th-century First Swedish Crusade and several subsequent incursions, undertaken by Scandinavian Christians against the then-pagan Finns—were dubbed "crusades" only in the 19th century, by romantic nationalist historians.

At the outset of the northern crusades, Christian monarchs across northern Europe commissioned forays into territories that comprise modern-day Estonia, Finland, Latvia, Lithuania, Poland, and Russia. The indigenous populations of Pagans suffered forced baptisms and the ravages of military occupation. Spearheading, but by no means monopolizing these incursions, the ascendant Teutonic Order profited immensely from the crusades, as did German merchants who fanned out along trading routes traversing the Baltic frontier.

The official starting point for the Northern Crusades was Pope Celestine III's call in 1195, but the Catholic kingdoms of Scandinavia, Poland, and the Holy Roman Empire had begun moving to subjugate their pagan neighbors even earlier (see Christianization of Pomerania). The non-Christian people who were objects of the campaigns at various dates included:

Armed conflict between the Finnic peoples, Balts, and Slavs who lived by the Baltic shores, and their Saxon and Danish neighbors to the north and south, had been common for several centuries before the crusade. The previous battles had largely been caused by attempts to destroy castles and sea trade routes to gain an economic advantage in the region, and the crusade basically continued this pattern of conflict, albeit now inspired and prescribed by the Pope and undertaken by Papal knights and armed monks.

The campaigns started with the 1147 Wendish Crusade against the Polabian Slavs (or "Wends") of what is now northern and eastern Germany. The crusade occurred parallel to the Second Crusade to the Holy Land, and continued irregularly until the 16th century.

The Swedish crusades were campaigns by Sweden against Finns, Tavastians, and Karelians during the period from 1150 to 1293. The wars with the Eastern Orthodox Novgorod Republic also had a religious aspect.

The Danes are known to have made at least three crusades to Finland. The first mention of these crusades is from 1187, when crusader Esbern Snare mentioned—in his Christmas-feast speech—a major victory over the Finns. The next known crusades were made in 1191 and in 1202; the latter was led by the Bishop of Lund, Anders Sunesen, with his brother.