A deal to bring the 20-year-old to Anfield was agreed a month ago and has now been finalised, adding to the captures of Rickie Lambert and Adam Lallana from Southampton.
The Germany Under-21 international told the club's official website: "I am proud to be a part of the family and I am looking forward to playing here. I will give it my best.
"LFC is a great club and the fans are amongst the best in the world. I am looking forward to the first game and can't wait to step on the pitch for the first time at Anfield."
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'Andy Robertson has a decision to make as a player, where you either roll your sleeves up and give someone great competition or you accept decisions and say that your time is done.' Alan Shearer on the conundrum facing Arne Slot's Liverpool
