Maddison has not played for England since earning his one cap to date in 2019 but he has been in brilliant form for Leicester, scoring six times in the Premier League this season.
The midfielder has been out of the England picture since he withdrew from a squad in October 2019 due to illness and was later pictured in a casino.
But Southgate said Maddison is “playing as well as any of the attacking players in this country” and offers “something slightly different to the other attacking players” in the England squad.
Explaining his decision, Southgate said: “He has earned the right. He is playing really well. He is a good player. We think he can give us something slightly different to the other attacking players that we have got.
“We decided a couple of weeks ago that we would [pick him]. I think he is playing as well as any of the attacking players in this country.”
Asked what Maddison’s reaction was, Southgate said: “He was delighted, I had some difficult calls which, emotionally, where at the other end of the spectrum, so it was nice to give myself an enjoyable [conversation].”