"Some of his play in the second half of the season was phenomenal and, if you look at our goals ratio as a team after he joined, you can see the impact he made.
"He is a team player, we were delighted with the way he performed in the Premiership and are looking forward to working with him at Celtic."
Fortune was officially unveiled on Thursday ahead of Celtic's trip to Australia for Sunday's opening pre-season friendly against Brisbane Roar.
And he is relishing the prospect of kicking off the next chapter in his football career, which began at Lille.
"I am looking forward to the challenge of winning the league title, competing for the cups and playing in Europe, in front of the Celtic supporters.
"I am also looking forward to working with my manager, Tony Mowbray, once again.
He can help our attacking midfield players score goals
Celtic manager Tony Mowbray
"I had a great experience with him, Mark Venus and Peter Grant in the Premiership last season and I am looking forward to now working with him at one of Europe's biggest football clubs."
Mowbray added that Fortune would give his Celtic side additional firepower.
"First and foremost he's a centre forward and his job is to try to give the team a cutting edge," the Celtic manager said.
"I think Marc does that but he also helps the attacking midfield players. He definitely did that at West Brom.
"He can help our attacking midfield players score goals."
Nancy midfielder Landry N'Guemo is also expected to join Celtic on a season-long loan.
The 23-year-old defensive midfielder, who is a Cameroon international, was wanted by Mowbray at West Brom last season.
Mowbray said a deal has yet to be finalised and that the player would not travel with the squad to Australia - despite Nancy claiming on their website that N'Guemo was now a Celtic player.
Fortune and N'Guemo would be ready-made replacements for two players not offered new deals by the new manager.
Jan Vennegoor of Hesselink and Paul Hartley, who signed for Bristol City on Monday, were told they were surplus to requirements as Mowbray looks to put his imprint on the squad.
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