The 22-year-old completed a medical on Merseyside on Tuesday and has become Liverpool’s second confirmed arrival of the summer for a £4.7m fee.
Jurgen Klopp was seeking some more competition for No 1 ‘keeper Simon Mignolet, and former German U21 international Karius arrives after keeping nine clean sheets for his club in the Bundesliga last season.
“It’s a very good feeling and it’s an honour to play for a club like this,” Karius, who has been allocated the No 1 jersey, told Liverpool’s website.
“They have a special history and the fans are amazing here, so I look forward to playing at Anfield.
“I know a lot about the club from watching them on TV. The history, everybody who plays football knows. There a lot of things in my head when I think of this club.
“I spoke to the manager and I had a good feeling afterwards because he told me what he wants to do with the club and with the players.
“It was a good talk with him and after that I was convinced this was the right decision.”
Karius is keen to challenge Mignolet for starts as soon as possible, and he is confident he has the ability to be a regular in the Premier League.
“I wouldn’t make the move otherwise if I thought I was just going to sit on the bench,” he added. “I want to play and I feel like I will be able to do that if I do well in training and pre-season and the pre-season games.
“I’m confident. I’ve played almost 100 league games in Germany, so I’m young but I’m experienced. I’m not afraid of anything. I want to be in goal and play for Liverpool every week and then win a trophy with the club, that’s what everyone wants. I want to do my part and help.”
Karius joins Schalke midfielder Joel Matip as a new arrival at Anfield – he agreed to join Liverpool in February when his contract expires this summer.
And Jurgen Klopp says Liverpool beat off competition from a number of other clubs to sign Karius, who spent part of youth career with Manchester City before joining Mainz in 2011.
“I am delighted we have signed Loris, he has had a fantastic season with Mainz and has developed into a very good goalkeeper since his return to Germany,” he said.
“I am really pleased that we were able to move so quickly to get him and that Loris has shown such a desire to come to Liverpool with a lot of other clubs interested in him.