Baker, who is currently playing on loan for Vitesse, last featured for the team in a European playoff against Croatia in October 2014.
Southgate has picked a 23-man squad for a Euro qualifier against Kazakhstan, which will be held on October 13 at the Ricoh Arena in Coventry.
The majority of players who performed in the side's wins over USA Under-23s and Norway last month have been selected in the latest squad.
England Under-21 squad:
Goalkeepers: Angus Gunn (Manchester City), Jordan Pickford (Preston North End, loan from Sunderland), Christian Walton (Bury, loan from Brighton & Hove Albion)
Defenders: Calum Chambers (Arsenal), Eric Dier (Tottenham Hotspur), Brendan Galloway (Everton), Joe Gomez (Liverpool), Kortney Hause (Wolverhampton Wanderers), Dominic Iorfa (Wolverhampton Wanderers) Jack Stephens (Middlesbrough, loan from Southampton), Matt Targett (Southampton)
Midfielders: Lewis Baker (Vitesse Arnhem, loan from Chelsea), Nathaniel Chalobah (Napoli, loan from Chelsea), Jake Forster-Caskey (MK Dons, loan from Brighton & Hove Albion), Ruben Loftus-Cheek (Chelsea), James Ward-Prowse (Southampton)
Forwards: Chuba Akpom (Hull City, loan from Arsenal), Jordon Ibe (Liverpool), Solly March (Brighton & Hove Albion), Nathan Redmond (Norwich City), Duncan Watmore (Sunderland), James Wilson (Manchester United), Cauley Woodrow (Fulham)