Southgate's previous deal was set to expire after next year's World Cup in Qatar but he has now pledged his future to the national team for a further two years.
The coach led England to the Euro 2020 final last summer and also guided the team to the last four of the 2018 World Cup in Russia.
Southgate said: "It remains an incredible privilege to lead this team. We have a great opportunity in front of us.
"I would like to take this opportunity to thank the players and support team for their hard work.
"I know they and the fans are all excited about what this squad could achieve in the future."
Southgate, who took over as England boss in 2016, has now set his sights on winning the nation's first major tournament since 1966 at the World Cup next year.
He said: "Over the last few months I've managed some of the biggest games in world football.
"There are no shortcuts to those moments. When you have a good team and have done a lot of work culturally, where they are a good team, you want to see it through and have a go.
"It's difficult to win major tournaments, we've only won one World Cup, but we have to believe that's possible. It's an aim we have as a team."
England demolished San Marino 10-0 in their last match to book their place in Qatar and Southgate is looking forward to making "new memories" with the national team.
He said: "England has been a massive part of my life as a supporter, player, under-21 manager and now manager.
"The tournaments and big matches are what inspired me as a kid and to be able to connect people to that has been very, very special - and the team can still improve and want to improve.
"To step away at a moment where the next few years could be very exciting would have stayed with me. I was very happy to commit to a project I really believe in."