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: “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,” he added.
— Alex Ox-Chamberlain (@Alex_OxChambo) August 31, 2017
“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.”