The 26-year-old Moroccan international, who spent last season on loan at the Britannia Stadium and scored a spectacular last-gasp winner against Chelsea, has become the seventh player to check-in at the Britannia Stadium during this transfer window.
The Potters have also landed Mame Diouf, who scored Saturday's historic match-winner against Manchester City, Steve Sidwell, Phil Bardsley, Bojan, Victor Moses and Dionatan Teixeira.
Stoke made a move for Assaidi after Peter Odemwingie suffered a knee injury in the 1-0 win over the reigning Premier League Champions at the weekend.
Chief Executive Tony Scholes said: "We thought we had concluded our incoming transfer business for this transfer window but we re-evaluated our plans after losing Peter to injury for the foreseeable future."
Manager Mark Hughes added: "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.
""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 Premier League appearances for the Potters last season and also scored against Newcastle and Everton and in a Capital One Cup tie against Birmingham.
The former Heerenveen man will report for training at Clayton Wood on Wednesday when the Potters' first team squad report back for training after a few days off following their stunning victory at the Etihad.