Liverpool have completed the signing of Alex Oxlade-Chamberlain from Arsenal in a deal believed to be worth up to £35million.
The Merseyside club have announced the 24-year-old England midfielder has agreed a long-term contract, understood to be five years, at Anfield.
Liverpool moved for Oxlade-Chamberlain this week after the player reportedly turned down the chance to join Chelsea.
Oxlade-Chamberlain, who is currently on international duty, underwent a medical at England's St George's Park training base after agreeing terms with the Reds.
Speaking from the England camp, where he is preparing for World Cup qualifiers against Malta and Slovakia, Oxlade-Chamberlain told the club's website, www.liverpoolfc.com: "I'm delighted to have signed for Liverpool.
"I'm not going to say too much today. I'm here with England and we've got two big games coming up, so I want to focus on them. But I can't wait to get to Melwood."
The former Southampton youth player, who has won 27 England caps, joined Arsenal in a deal worth a reported £15million in 2011.
In a farewell message to Arsenal, Oxlade-Chamberlain wrote on Instagram: "After six years, three FA Cup wins and countless incredible memories, the time has come for me to move on from Arsenal.
"I want to publicly thank Arsene (Wenger), the coaching and club staff and my amazing team mates for all of the incredible opportunities I have been given.
"Thank you so much to you, the fans, who have always had my back - I honestly couldn't be more grateful. The club, and my time here, will always mean a massive amount to me.
"I know my choice might come as a surprise to many, and the decision to leave was tough after being a part of the club for so many years, but I feel that this move is right for the next stage in my ongoing development."
The deadline-day deal is Liverpool's second signing of the week after an agreement to bring in midfielder Naby Keita from Red Bull Leipzig next summer was completed on Tuesday.
He is also the club's fourth major arrival of this summer's window after Mohamed Salah, Dominic Solanke and Andy Robertson.
The Gunners had hoped to keep Oxlade-Chamberlain but his reluctance to re-sign forced them to cash in now to avoid losing him on a free transfer next summer.
His final Arsenal appearance was in their 4-0 hammering by Liverpool at Anfield last Sunday.
Reds boss Jurgen Klopp said: "I am really, really happy we have got Alex signed. Since the last game I have been waiting and hoping we could make this transfer - and when I got the news he was ours it was fantastic.
"He may still be young but he has a lot of experience in the Premier League, Champions League and for his country. Already he is a very, very good player and I think he has the mentality and desire to get even better - and that excites me.
"This is someone making a tough decision to leave one great club and come to another great club and I think he had many options, so to get him means I am delighted.
"He didn't make an easy decision but I think he made a great one to go on this journey with this squad of talented boys and our great supporters. I welcome him to the Liverpool family."
Liverpool, meanwhile, remain adamant playmaker Philippe Coutinho will not be sold to Barcelona.
The club have rejected three bids - the most recent reportedly worth up to £118million - for the Brazilian.
Reports in Spain have claimed Barca have moved again for the 25-year-old but Liverpool - who said earlier this month their "definitive stance is that no offers for Philippe will be considered" - insist their position has not changed.
Liverpool also continue to be linked with a deadline-day move for Southampton defender Virgil van Dijk despite his club's insistence he is not for sale. A potential move for the Dutchman collapsed in June following a complaint by Saints over the Anfield club's conduct.
Monaco winger Thomas Lemar, also linked with Arsenal, is another player reportedly on the club's radar.