Oxlade-Chamberlain has put pen to paper on a five-year deal that will see him earn £120,000 a week.
The 24-year-old was entering the final year of his Gunners contract and reportedly rejected a new deal worth up to £180,000 a week to stay at the Emirates.
Oxlade-Chamberlain turned down the opportunity to join Antonio Conte's Chelsea as he wanted to play a more central role in the midfield, something he felt Jurgen Klopp could provide.
Arsene Wenger has used the England international as a wing-back in all of Arsenal's Premier League games this season.
The Ox leaves the Gunners after making 198 appearances and scoring 20 goals since joining from Southampton in August 2011.
He becomes Liverpool's fourth major signing of the summer after the arrivals of Mohamed Salah, Andrew Robertson and Dominic Solanke.
Welcome, @Alex_OxChambo! pic.twitter.com/joYm7hvK3m
— Liverpool FC (@LFC) August 31, 2017
Oxlade-Chamberlain told Liverpool’s official website:
“Firstly, I’m delighted to have signed for Liverpool and a big thank-you to everyone here at the FA that has made this possible, using the facilities. It has been a massive help.”
“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.”
Manager Klopp also added:
“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.”
“I hear a lot of talk about positions but let’s talk about the player; he is someone with really good abilities who always gives everything for his team. A player that is positive and willing to take risks to try and make positive things happen.
“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.”
Reports suggest that the Reds transfer dealings may not be over as they look to conclude a deals for Monaco's Thomas Lemar and Southampton's Virgil van Dijk.