The Morocco international spent the 2013-14 season on loan at the Britannia Stadium, scoring a spectacular last-minute winner against Chelsea, and becomes the club's seventh player to sign for the club this summer.
Mark Hughes has also acquired Mame Biram Diouf, Steve Sidwell, Phil Bardsley, Bojan, Victor Moses and Dionatan Teixeira in the transfer window and moved for Assaidi after Peter Odemwingie picked up a knee injury during the club's 1-0 win over Manchester City at the weekend.
“It’s unfortunate that we look like being without Peter for a while but following his setback we felt it was important that we moved to bolster the resources at our disposal," Hughes told the club's official website upon completion of Assaidi's move.
“It’s clear from talking to Oussama that he really enjoyed his time with us last season and he’s looking forward to picking up where he left off with us.”
Assaidi made 19 appearances for Stoke last season and could make his second debut for the club against Leicester following the international break.