Nikica Jelavic was signed from Everton for an initial £6.5million, rising by an additional £1million should the Tigers stay in the Barclays Premier League, and the undisclosed deal for Republic of Ireland striker Long is understood to be worth a similar amount.
West Brom agreed a fee for Long on Thursday, with the player having a medical with Hull on the same day.
Personal terms were finalised on a three-and-a-half-year deal on Friday morning and Long's paperwork was cleared in time for the 26-year-old to be available for the Barclays Premier League trip to Norwich.
Long, who joins fellow Republic of Ireland internationals David Meyler, Robbie Brady, Paul McShane and Stephen Quinn in East Yorkshire, said: "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.
"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."
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