The 28-year-old Turkish midfielder has penned a three-year deal at the club - following his release by Spanish side Celta Vigo at the end of the season.
Baggies boss Steve Bruce said he is delighted at the intent showed by the club to bring in a player of Yoksulu's talent on top of both Jed Wallace and John Swift.
He said: “I am delighted we have been able to secure a player of Okay’s quality. Having already been able to sign Swifty and Jed this window, I’m really pleased the club have shown further intent by bringing in Okay.
“The middle of the park is an area in which I felt we needed more quality and from what I have seen of him, Okay certainly offers that in abundance. I know he is a fans’ favourite following his first spell at the club and I’m confident he will shine for us again this season.
“I must applaud the club for reacting quickly to an ever-changing situation. Ron and the staff behind the scenes have worked tirelessly to seal Okay’s signature.
“We faced fierce competition from some big clubs across Europe, but the player made it clear he wanted to return to the Albion, and he has his sights set firmly on the Premier League. We really are delighted to have him.”
Yokuslu joined Albion on loan from the Spanish side in January 2021 - as then Baggies boss Sam Allardyce set about trying to help the club avoid the drop.
The midfielder played a pivotal role in the run in - turning out 16 times for Albion and putting in some memorable displays.
However, he couldn't prevent Albion slipping back into the Championship and he returned to Celta Vigo that summer.
He then joined Getafe on loan last season - before his contract with Celta ran out this summer.
That prompted speculation that the midfielder could be set for a sensational return to the club - however Bruce told the press earlier this month than he believed that 'the ship had sailed' on the midfielder.
However, the situation changed and last week the Baggies boss insisted they were in with a chance of signing the midfielder.
Yokuslu will now arrive at the club on receipt of a work permit.