The original parish probably dates back to the early Middle Ages; Until the 12th century, most episcopal churches were dedicated to Peter. In 1299, a chapter with twelve secular canons was attached to the church, after Walram de Rosse van Valkenburg had submitted a request for this a year earlier. This request may have had both religious and political motives. After Walram had obtained control of Sittard and the surrounding area, he regarded this place as the center of his territory. The founding of a chapter would give the city more prestige.
Whether Walram de Rosse intended the collegiate church as a future burial church for himself and his dynasty has not been proven, although according to tradition he would have been buried in the predecessor of the current church. Walram and some of his successors organized various annual services at the collegiate church. Part of the income from the churches of Sittard, Munstergeleen and Susterseel, churches over which Walram had guardianship, went to the chapter. In 1336, the church of Merkelbeek was also assigned to the chapter by the Liège Prince Bishop Adolf van der Mark.
The oldest parts of the church are built of brick. This may have been done to save costs. This applies to the current ship, which must date from around 1400 (according to some sources from around 1350). Parts of the wallwork of the transept are also made of brick. Later, marl was mainly used. In the 15th century the choir was rebuilt and expanded with two bays and an apse.
A deed from 1505 shows that the church tower dates from this period. In the 16th century, the Lady Chapel at the northern transept was also raised.