Fortune netted five goals in 18 appearances for the Baggies on loan from Nancy in the second half of their last Premier League campaign in 2009 before being reunited with Tony Mowbray after the manager moved to Celtic Park.
Fortune struggled to live up to his £3.8million price tag, although he was not helped by a knee injury early in his Parkhead career, and failed to win a trophy.
He netted 12 goals in 43 matches and began to win fans over with his all-round play but he has become the latest victim of Neil Lennon's rebuilding job and completes a move back to the Black Country for a reported fee of £2.5million.
The French-Guyana forward has been registered in time to play in Sunday's clash at Liverpool, subject to international clearance, and West Brom manager Roberto Di Matteo is delighted.
He told the club's official website: "I'm delighted to welcome Marco back to the club.
"The great thing about this signing is that it won't take him long to settle in because he knows the club inside out. He also knows what this league is all about.
"Marco got on really well with the other players during his loan here and they're all delighted to see him back.
"He is a powerful striker who can hold the ball up and bring others into play and having also signed Peter Odemwingie last week, we now have plenty of competition up front."
Fortune joins former Lokomotiv Moscow forward Odemwingie, who hit the winner against Sunderland last weekend, in signing for the club this summer alongside Boaz Myhill, Nicky Shorey, Gabriel Tamas, Somen Tchoyi, Pablo Ibanez and Steven Reid.