The 22-year-old central midfielder, who was in the stands for last night's Europa League match with Gomel, has signed a "long-term contract" with the Reds after successfully completing his medical.
Allen told the club's official website:
I feel incredible. Everyone knows the history of this club, it's a massive club, and I'm delighted to have joined.
The passion that people here have for football is something I share and I want to be part of that.
I'm looking forward to being part of some great years ahead for Liverpool Football Club.
His manager, Brendan Rodgers, was equally as happy with the transfer, saying:
I'm absolutely delighted that Joe has made the decision to come with us on this journey.
Joe is a player whose profile will fit perfectly with the ideas of this group. His ability to control and dominate the ball is an important ingredient in our attempt to gain success on the field.
Joe has had a fantastic education at Swansea City and will now begin the next chapter in his exciting career, and I wish him all the very best in what I'm sure will be a long and distinguished career here at Liverpool.
It's no wonder the former Swans boss is happy to have finally got his man, as now he can kickstart his footballing revolution at the club.
And it's that style of attractive football—the new "must-have" in the sport, thanks to Spain and Barca—which persuaded Liverpool's owners to part ways with the antiquated ideas of Kenny Dalglish and bring in the chic new thinking of Rodgers.
But to get that kind of football flowing, the new boss needed passing footballers.
In his first competitive matches, the two Europa League third-round qualifying games against Belarusian side Gomel, the signs of change were there (reflected in the 67 percent possession in the return leg at home), but it was nonetheless clear the players would need time to adapt.
The signing of Allen, however, has sped up the buffering, giving Rodgers a Liverpool 2.0 just in time for the new Premier League season.
While the current Reds crop are all adjusting to a style of play where building play gradually from the back, waiting for the right chance and retaining possession are most important—only three Liverpool players hit more than 50 passes per game, only one had a pass success rate over 90 percent—the acquisition of Team GB star Allen means Rodgers will have at least one established, reliable and accurate passing source who'll virtually never give away the ball.
Last season in the Premier League, Allen hit on average 60.5 passes with a success rate of 91.2 percent—better than any Liverpool player—and also hit on average 5.4 accurate long balls per game—better than any outfield Reds star.
It's that sort of almost Xavi-esque accuracy—his ability to duck under markers and keep the ball under pressure is also comparable to the Spain, Barcelona and world football legend—which is vital to Rodgers' plans and finally gives him the necessary player to get the keep-ball game going.
And though Joe Allen is the type of player the Reds need for their new tactics, a lurking question has been, why him? After all, £15 million is huge money for a player who's only 22, has spent just one season in the top flight, scored just four goals and made only two assists in the last campaign.
The truth is, there was no other player like Allen available on the market for Liverpool.
But none fit the bill quite like Allen, a player who's hit more passes and with a better success rate, as well as more accurate long balls, than the aforementioned players.
Plus, his relationship with the manager he played for last season means there's unlikely to be a dip in such form, while, at 22, Allen is younger than the Reds' other transfer targets and so should have more potential and a better resale value.
In essence, Allen was the best Liverpool were ever going to get, and now that he's over the line at Anfield, his signature is a big boost to the club's future plans.
It's expected he'll form a midfield three with Lucas Leiva and Steven Gerrard (h/t goal.com), with Allen and Lucas performing roles akin to Michael Carrick and Paul Scholes at Manchester United—Lucas the Carrick-esque holding midfielder, doing most of the defensive work to allow the Allen, the Scholes-esque deep-lying playmaker to pull the midfield strings and dictate the team's play.
A key signing for Liverpool, the purchase of Joe Allen will finally allow Brendan Rodgers to kickstart his Anfield revolution.
All statistics in this article are courtesy of WhoScored.com