The 28-year-old Mali forward scored what turned out to be the winner on the hour mark in the Primeira Liga clash against his former club, and celebrated by pointing to his forearm.
This seemingly led to the home fans making monkey chants and throwing chairs at the player.
Porto head coach Sergio Conceicao tweeted after the game to say: "We are all Moussa #notoracism."
The club also issued a statement strongly backing their player in the fight against racism.
It read: "Following the abuse suffered by the player Moussa Marega during the Vitoria Guimaraes v Porto game, Porto announce that:
"1. The entire structure of Porto and its supporters are in solidarity with Moussa Marega, who was led to take drastic action in the wake of repeated racist insults.
"2. Porto strongly condemns racist behaviour this afternoon, which is one of the low moments in the recent history of Portuguese football and will have to be properly penalised.
"3. Porto will remain at the forefront of the fight against racism and hate crimes, and its team maintains the will to continue fighting them on the field."