The Moroccan international spent last season at the Britannia Stadium and has returned as cover for the injured Peter Odemwingie, with a loan fee of around £1million attached.
Potters manager Mark Hughes had been interested in Assaidi earlier in the window but only returned for a player who scored five times in 25 appearances last year after Odemwingie suffered a knee problem.
Hughes told Sky Sports News: "Ozzy was someone we were interested in earlier in the window so we're delighted with that. You never give up on these things, there was still an opportunity so we're delighted to get it over the line.
Chairman Peter Coates told BBC Radio 5 Live: "He was with us last year and we liked him very much.
"We didn't think we'd be doing any business if it wasn't for Peter's injury. But we work hard to make sure we've got options if something happens at short notice.
"We were fortunate in a sense that Assaidi was on our radar and we'd had him before."