The Bosnian Church (Serbo-Croatian: Crkva bosanska, Црква босанска) was an autonomous Christian church in medieval Bosnia and Herzegovina.

Historians traditionally connected the church with the Bogomils, although this has been challenged and is now rejected by the majority of scholars. Adherents of the church called themselves simply Krstjani ("Christians") or Dobri Bošnjani ("Good Bosnians"). The church's organization and beliefs are poorly understood, because few if any records were left by church members and the church is mostly known from the writings of outside sources — primarily Catholic ones.

The monumental tombstones called stećak that appeared in medieval Bosnia, as well as Croatia, Serbia, and Montenegro, are sometimes identified with the Bosnian Church.

Christian missions from Rome and Constantinople were sent into the Balkans in the 9th century, Christianizing the South Slavs, and establishing boundaries between the ecclesiastical jurisdictions of the See of Rome and the See of Constantinople. The East–West Schism then led to the establishment of Catholicism in Croatia and most of Dalmatia, while Eastern Orthodoxy came to prevail in Serbia. Lying in-between, the mountainous Bosnia was nominally under Rome, but Catholicism never became firmly established due to a weak church organization and poor communications. Medieval Bosnia thus remained a "no-man's land between faiths" rather than a meeting ground between the two Churches, leading to a unique religious history and the emergence of an "independent and somewhat heretical church".

Catholicism and Eastern Orthodoxy predominated in different parts of what is today Bosnia and Herzegovina; the followers of the former formed a majority in the west, the north, and in the center of Bosnia, while those of the latter were a majority in most of Zachlumia (present-day Herzegovina) and along Bosnia's eastern border. This changed in the mid-13th century, when the Bosnian Church began eclipsing the Roman. While Bosnia remained nominally Catholic in the High Middle Ages, the Bishop of Bosnia was a local cleric chosen by Bosnians and then sent to the Archbishop of Ragusa solely for ordination. Although the Papacy already insisted on using Latin as the liturgical language, Bosnian Catholics retained the Church Slavonic language.

In 1199 the ruler of Duklja, Vukan Nemanjić, wrote to Pope Innocent III that Bosnian ruler Kulin and his family and 10,000 other Bosnians had become heretics. The Archbishop of Spalato, vying for control over Bosnia, joined Vukan and accused the Archbishop of Ragusa of neglecting his suffragan diocese in Bosnia. Emeric, King of Hungary, a supporter of Spalato, also seized this opportunity to try to extend his influence over Bosnia. Further accusations against Kulin, such as harbouring heretics, ensued until 1202. In 1203, Kulin moved to defuse the threat of foreign intervention. A synod was held at his instigation on 6 April. Following the abjuration of Bilino Polje, Kulin succeeded in keeping the Bosnian Diocese under the Ragusan Archdiocese, thus limiting Hungarian influence. The errors abjured by the Bosnians in Bilino Polje seem to have been errors of practice, stemming from ignorance, rather than heretical doctrines.

The bid to consolidate Catholic rule in Bosnia in the 12th to 13th centuries proved difficult. The Banate of Bosnia held strict trade relations with the Republic of Ragusa, and Bosnia's bishop was under the jurisdiction of Ragusa. This was disputed by the Hungarians, who tried to achieve their jurisdiction over Bosnia's bishops, but Bosnia's first Ban Kulin averted that. In order to conduct a crusade against him, the Hungarians turned to Rome, complaining to Pope Innocent III that the Kingdom of Bosnia was a centre of heresy, based on the refuge that some Cathars (also known as Bogomils or Patarenes) had found there. To avert the Hungarian attack, Ban Kulin held a public assembly on 8 April 1203 and affirmed his loyalty to Rome in the presence of an envoy of the Pope, while the faithful abjured their mistakes and committed to following the Catholic doctrine. Yet, in practice this was ignored. After the death of Kulin in 1204, a mission was sent to convert Bosnia to Rome but failed.

On 15 May 1225, Pope Honorius III spurred the Hungarians to undertake the Bosnian Crusade. That expedition, like the previous ones, turned into a defeat, and the Hungarians had to retreat when the Mongols invaded their territories. In 1234, the Catholic Bishop of Bosnia was removed by Pope Gregory IX for allowing supposedly heretical practices. In addition, Gregory called on the Hungarian king to crusade against the heretics in Bosnia. However, Bosnian nobles were able to expel the Hungarians once again.