Liverpool's Trent Alexander-Arnold is the most notable inclusion in an England World Cup squad which stands out for its youthfulness.
The 19-year-old was celebrating a maiden senior call-up on Wednesday as Adam Lallana, Joe Hart and Jack Wilshere all missed out.
An impressive performer in Liverpool's run to the Champions League final, Alexander-Arnold briefly trained with England in March and has now been named in the senior squad for the first time.
Meanwhile, England's three keepers for Russia 2018 - Jack Butland, Jordan Pickford and the uncapped Nick Pope - have won just nine caps between them, while midfielder Ruben Loftus-Cheek has made just two Three Lions appearances.
"I believe this is a squad which we can be excited about," Southgate said.
"It is a young group, but with some really important senior players so I feel the balance of the squad is good, both in terms of its experience, its character and also the positional balance.
"The first call-up for Trent Alexander-Arnold is well deserved. When we pick young players, it's not just because they are young, it's because their performances deserve it."
Alexander-Arnold revealed he had been informed of his inclusion by Liverpool boss Jurgen Klopp as he prepared for a training camp in Spain.
"I found out this morning, just before we came out to Marbella," he told the club's official website.
"The manager came and spoke to me just as we were getting on the plane.
"He asked me if I knew anything about the squad or anything yet. I said, 'no' and he said, 'Have you got anything planned around the time of the World Cup?'. I was like, 'No, no holidays or anything...'.
"He said to me, 'OK, good... You're in the squad!'. It was a really proud moment. On England's behalf, it was nice of them to let the manager tell me."
Southgate has made 12 changes to the squad that floundered at Euro 2016 under Roy Hodgson, underlining his desire to build a group capable of providing long-term success - even if that means more experienced players missing out.
Other controversial calls are the decision to include Arsenal's Danny Welbeck and Chelsea defender Gary Cahill - while leaving the likes of Wilshere and Hart at home.
England have warm-up friendlies against Nigeria at Wembley on June 2 and Costa Rica in Leeds on June 7, with the Group G opener against Tunisia on June 18.