The Manchester United shot stopper has joined up with the Three Lions for Euro 2024.
However, the 38-year-old is not going to play games, he will only be a supporting reserve.
Southgate said: "I am really pleased Tom has agreed to join us in Germany.
"Experience tells us that a tournament can put a lot of demand on our goalkeeping group led by Martyn Margetson.
"Tom will play an invaluable role on the training pitch supporting Jordan, Aaron and Dean.
"Off the field, as someone we all know well, he will add to the positive environment we are always looking to create.
"We are grateful to Manchester United for their support in making this happen."
