The two clubs have agreed an initial sixth-month loan deal for the 28-year-old with a compulsory purchase option that will keep him at Strasbourg for another two years.
Waris had been at Porto since an initial loan that began in January 2018 but failed to establish himself at the club and spent last season on loan in France with Nantes.
“It was my priority to come back to France. Strasbourg is a club I know,” Waris told the club’s website.
“I recently faced Strasbourg with FC Nantes. There is an incredible atmosphere in the stadium and I want to live great moments with the fans.
“I hope to bring my qualities to a team that already has very good strikers.”