The Republic of Ireland striker passed a medical on Thursday and completed the formalities of his transfer on Friday morning, in time to feature against Norwich this weekend.
Long joins on a three-and-a-half-year deal in the same week Hull spent a reported £6.5m on Everton striker Nikica Jelavic.
His arrival takes Steve Bruce's January spending on strikers well beyond the £10m mark and adds further goal threat to a squad which has managed just 22 in 21 Premier League games this season.
"I finally got the deal signed," Long told the club website. "I've met all the lads 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's fellow Irishmen Paul McShane and Robbie Brady are already at the KC Stadium, and he said: "I'm lucky I've had a few inside informers letting me know what it's like up here. They all love it, 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."
The transfer ends Long's two-and-a-half years at Albion, where he scored 19 goals in 81 league games.