Defoe, who last played for England in a 2-2 draw with Scotland last summer, has earned 57 caps for the national team in his career.
The Cherries striker recently recovered from an ankle injury, and told the Daily Mail that he is prepared to fight for a place in Southgate's team this summer.
"I spoke to Gareth and he basically said, 'The door is always open because you bring so much with your experience, around the place with the lads you bring a lot to the camp. Just get yourself back in the team, get your goals and you know the door is open,'" Defoe said.
"Realistically, if I do end up going, it will be my last one. I remember the feeling I got from going to my first - just knowing that I'm at a World Cup, something I've worked so hard to achieve and knowing that I've done it.
"If I can go to this one, it will be special, especially with that group of young players. At a tournament, it's always about the squad. When you are not playing, you've always got to be switched on."
England play the Netherlands and Italy in warm-up games later this month.