Folly Returns, Whelan Listed
The 21-year-old midfielder arrives at S6 for an undisclosed fee, becoming the Owls' fourth signing this summer.
Yoann made 14 appearances for SWFC towards the end of last season.
Wednesday boss Paul Sturrock told the club's official website: 'I'm very pleased. Yoann is a manager's type of player.
'He does everything effectively, is a good passer and has a knowledge about the game.'
However, Folly's transfer has taken the Owls towards the top of their player budget for the coming season and, with a goalkeeper yet to sign, Glenn Whelan has been transfer listed.
Boss Paul Sturrock said: 'I was delighted to sign Yoann but it means we have to look at trimming the squad to keep us within budget.
'We have several options in midfield now, so it would be to the benefit of both Glenn and Sheffield Wednesday if he were to move on to find more regular first-team football.'