The Republic of Ireland international saw a move to the KC Stadium on deadline day in September break down, despite travelling to Yorkshire for a medical.
His arrival follows the signing of Nikica Jelavic, who completed a similar move from Everton on Wednesday.
Long hailed positive reports from friends within the Hull squad as key in his decision to move to Steve Bruce's side.
He said: "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 in the game, I was happy to start a new part of my career here."
Long - capped 42 times by the Republic of Ireland - scored 22 goals in 87 appearances for West Brom after joining from Reading in 2011.
Bruce had made no secret of his desire to sign striking reinforcements in January after seeing his side manage just 22 goals in their opening 20 games.
Robbie Brady netted three in Hull's first six matches and remains the club's top scorer - although a strong defence has helped Bruce's men to 10th place upon their return to the top flight.
Long and Jelavic's arrivals have been offset somewhat by the departures of Aaron McLean, who has joined Bradford City, and Gedo, who has had his loan from Al Ahly cancelled.
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