Maguire, a 24-year-old central defender, impressed last season with Hull City before making a move to Leicester during the close-season.
"I thought his performances for Hull were very good," said Southgate.
"He is a defender who is very comfortable with the ball, he has already adjusted very well to a new club at Leicester, plays with composure and can defend the ball into the box, so this is a good chance to have a look at him."
Chalobah, a product of Chelsea's youth system who has been a regular for England at age-group level, joined Watford recently after several loan spells during his time at Stamford Bridge.
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The squad announcement followed Wayne Rooney's decision on Wednesday to call time on his England career and Southgate confirmed he had spoken to the country's all-time top scorer.
"I called him, but he said he had been thinking long and hard about his decision and he announced his international retirement," said Southgate.
"I was thinking of including him in the squad. His form has been excellent. If you are the most capped outfield player and record goalscorer, I am not sure what else you have to do to be an England legend. He has served his country brilliantly and I congratulate him on a fantastic international career."
Southgate has also called up uncapped goalkeeper Jordan Pickford of Everton.
England squad:
Goalkeepers: Joe Hart (West Ham United), Jack Butland (Stoke City), Jordan Pickford (Everton), Tom Heaton (Burnley).
Defenders: Ryan Bertrand (Southampton), Aaron Cresswell (West Ham United), Gary Cahill (Chelsea), Phil Jones (Manchester United), Harry Maguire (Leicester City), John Stones (Manchester City), Michael Keane (Everton), Chris Smalling (Manchester United), Kyle Walker (Manchester City), Kieran Trippier (Tottenham Hotspur).
Midfielders: Eric Dier (Tottenham Hostpur), Nathaniel Chalobah (Watford), Jake Livermore (West Bromwich Albion), Jordan Henderson (Liverpool), Raheem Sterling (Manchester City), Alex Oxlade-Chamberlain (Arsenal), Dele Alli (Tottenham Hotspur), Jesse Lingard (Manchester United).
Forwards: Marcus Rashford (Manchester United), Danny Welbeck (Arsenal), Harry Kane (Tottenham Hotspur), Jamie Vardy (Leicester City), Daniel Sturridge (Liverpool), Jermaine Defoe (Bournemouth).
(Reporting by Simon Evans; Editing by Christian Radnedge/Mark Heinrich)
