Assaidi impressed in a similar move to the Britannia Stadium last season, making 19 appearances and scoring four goals.
The 26-year-old joined Liverpool from Dutch club Heerenveen in 2012 but he failed to make much of an impression on their first-team squad and made only four Premier League appearances before leaving on loan.
He was a target for Potters boss Mark Hughes throughout the summer and the Welshman told Sky Sports he was pleased the Morocco international would be coming back for a second spell at the club.
Hughes said: "We're delighted. We tried to get Ozzy earlier in the window but we've been able to do it and we're pleased.
"You never give up on these things and we are delighted to get it over the line. It's important we have cover and Ozzy is someone we know, somebody who did very well for us last season."
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 (Odemwingie) to injury for the foreseeable future.”
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.”