The parish church of the Assumption of the Blessed Virgin Mary was consecrated on August 15, 1638 by the bishop of Łuck, John the Baptist of Zamość. At the invitation of Michał Korwin Krassowski, the parish-priest of Siemiatycze, and prince Michał Józef Sapieha, the priests missionaries of Saint Vincent a Paolo arrived in the town and took over the parish on September 29, 1719. The first missionary parish priest was Kazimierz Józef Ottowicz. On July 3, 1722, a document was signed for the transfer of the parish,the church and other benefit to the convent of missionaries by Stefan Rupniewski, the bishop of the Lutsk diocese. In 1723, the monks built a parish school, an asylum and a hospital (now a parish room) according to Karol Antoni Bay's project. In 1719-1727, they built a monastry opposite the church front. They also rebuilt the facade of the church and the porch. In 1737, they built a tower. The builder was the son-in-law of Karol Antoni Bay, an Italian archictect, Wincenty Rachetti. In the years 1725-1727 they built a bell tower, which was incorporated into the stone fence and surrounding the church cemetery. During the city fire in 1758, the parish church was partially destroyed. As part of the reconstruction and expansion of the city, the destroyed church and the missionary monastery were renovated in 1777 by the then owner of Siemiatycze and Kock (1750-1800) - Princess Anna of Sapieha Jabłonowska. On December 4, 1793, King Stanisław August Poniatowski participated in a mass celebrated in the church.