Swans had a scout posted to see Marega in action against Benfica last weekend.
Marega has scored 20 goals in all competitions this season.
Everton and West Ham are also said to be in the hunt for the 26-year-old.
Swansea have also been trailing Porto teammate Vincent Aboubakar this season. The Welsh club had a deal in place for the Cameroon striker last summer, but instead opted to complete a deal to bring Wilfried Bony back to SA1 from Manchester City.