They signed a treaty with (the kingdom that would become) Spain called the Treaty of Tordesillas that defined the extent to which Portugal was allowed to colonise westwards, in exchange for limiting Spanish expansion in Africa and Asia. It gave Spain basically the entire Americas except for what is today the very eastern tip of Brazil. It was signed with papal approval.

Eventually it was expanded to add another "line" to define how far into Asia Spain was allowed to colonise from the west.

The limits and lines were in either case not very well defined and both Spain and Portugal ended up breaking them (plus neither France nor Britain nor any other colonial power was bound by them, apart from Spain and Portugal) pretty blatantly. But it did mean that the Carribean was off-limits to Portugal for as long as it was taken at least somewhat seriously.

Would you consider Brazil to be part of the "Caribbean region?" It was, of course, Portugal's major overseas colony in the Americas.