The founding of Belém can be traced back to the early 17th century when Portuguese expansion in South America necessitated the establishment of fortified posts along important waterways. Captain Francisco Caldeira Castelo Branco led the endeavor to create a settlement at the mouth of the Amazon River. His aspirations were fueled by the lucrative opportunities presented by the rich resources of the Amazon basin and the necessity for Portugal to assert its dominance in an area increasingly threatened by Spain and other European powers. The location chosen, at the confluence of the Guamá River and the Amazon, provided an excellent vantage point for overseeing maritime traffic and offered fertile grounds for future development.

When Caldeira arrived in January of 1616, the region was largely uninhabited aside from local Indigenous tribes. He and his crew set about constructing makeshift structures and fortifications to protect the nascent settlement from both natural elements and potential threats from rival colonial powers. The establishment of Belém was marked by the construction of a fort, known as Fort do Presépio, which not only served a military function but also symbolized the intent of the Portuguese crown to colonize and exploit the Amazon's resources.

Over the next few years, the settlement attracted other settlers, including farmers and traders, drawn by the promise of economic opportunities. The area grew as a commercial hub, facilitating trade with not just the local Indigenous communities but also with Europe. Belém’s founding was significant, as it laid the groundwork for future Portuguese settlements in the Amazon, contributing to the colonial expansion of the region and altering the cultural and economic landscape of the Amazon basin.