The Republic of Ireland international has agreed a three-and-a-half-year contract and becomes Steve Bruce's second transfer in January after Nikica Jelavic's arrival
The 26-year-old has scored three goals in 15 Premier League appearances this term and rejected two new contract offers from West Brom prior to his move to the KC Stadium.
“I finally got the deal signed. I’ve met all of the lads today, and everybody seems really friendly. It’s a club that’s on the up, and I’m delighted to be a part of that,” Long told the club's official website.
“I’m lucky that I’ve had a few inside informers letting me know what it’s like up here. They all love it up here, and they’re excited by what is happening at the club.
“After meeting the manager, and respecting what he’s done as a player and a manager, I was happy to start a new part of my career here and hopefully I can score a few goals to take us further up the league.”
Long becomes West Brom's second signing of the January transfer window following the arrival of Croatian striker Nikica Jelavic, who joined from Everton on Wednesday.