First Swedish Crusade is a legendary military expedition presumably in the 1150s that has traditionally been seen as the conquest of Finland by Sweden, with pagan Finns converted into Christianity. According to the legend, the crusade was conducted by king Eric IX of Sweden. Bishop Henry of Uppsala accompanied him and remained in Finland later to become a martyr there.
The actuality of the crusade is debated amongst academics. No archaeological data give any support for it, and no surviving written source describes Finland under Swedish rule before the end of 1240s. Furthermore, the diocese and bishop of Finland are not listed among their Swedish counterparts before the 1250s.
At the time, leading the ledung was the responsibility of the jarl. This has resulted in a theory that Eric conducted the expedition before he became the king, or at least a pretender for the throne. Legends give no year for the expedition, and attempts to date it to an exact year in the 1150s are all much later speculations. All that is known about king Eric and bishop Henry is that they most probably held important positions in Sweden sometime in the mid-12th century.
Worth noting is also the fact that the Swedish bishop normally involved in the eastern campaigns was the Bishop of Linköping, not the Bishop of Uppsala.
The mid-12th century was nevertheless very violent time in the northern Baltic sea, with Finns and Swedes in frequent conflicts with Novgorod. As a part of that, a Swedish military expedition may have taken place also against Finland. Noteworthy is especially the short story in the First Novgorod Chronicle that in 1142 a Swedish "prince" and bishop accompanied by a fleet of 60 ships plundered just three Novgorodian merchant vessels somewhere "on the other side of the sea", obviously being after something more important.