The 36-year-old was awarded a two-year deal after his previous contract expired in the summer and the German goalkeeper now looks forward to challenging the club's other glovemen Lukasz Fabianski and Kristoffer Nordfeldt for a starting spot.
He told the club's official website: "It was really nice of the club to allow me to train with the U21s and then the first team, despite me not having signed a contract.
"Garry (Monk, manager) wants strong competition in every spot and, when you look at the players we have pushing for each position, you can see we have that.
"We have Lukasz, Kristoffer and now myself in the goalkeeping department, and I think that's a great situation for the manager to have.
"We are here to push each other very hard in each training session, and that will help the club move forward.
"I have always said that I wanted to continue playing until I was 38 and then I would consider what I would do next.
"This contract will take me up to that age and I'll see what happens then."