Long-term mainstays such as goalkeeper Joe Hart and midfielder Jack Wilshere were not among the names chosen to travel to Russian Federation, but their absences were announced earlier in the week after they had been informed personally by Southgate.
However, a knee problem ruled him out of England's March friendlies against Holland and Italy and a final chance to impress Southgate, who plumped for Ruben Loftus-Cheek alongside his regular midfielders after the Chelsea youngster impressed on loan at Crystal Palace.
England head to Russian Federation next month hoping to improve on a dismal 2014 campaign in Brazil and an embarrassing performance at Euro 2016.
'I believe this is a squad which we can be excited about, ' said the England boss in a statement from the FA.
"I'd take him. His Champions League form this year has been nothing short of breathtaking", he said.More news: Trucker Putin opens Russian bridge to annexed Crimea
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Arsenal midfielder Jack Wilshere was left out of the squad having made the 30-man provisional list released a few days ago. 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...'
Alexander-Arnold has all that, and more. "When we pick young players, it's not just because they are young, it's because their performances deserve it", Southgate told The Guardian.
Coach Gareth Southgate expressed his pleasure with his selections.
England will begin their Group G campaign against Tunisia on June 18, before fixtures against Panama on June 24 and Belgium on June 28.
"Dreamt of going to a World Cup since I was a kid", tweeted Alexander-Arnold, who - to some - is still is a kid.
Southgate named his 23-man squad on Wednesday and chose to omit Wilshere, who was also left off the five-man standby list for the upcoming tournament in Russian Federation.