The Cherries striker netted a late equaliser against Watford on Saturday, just the fourth goal of an injury-hit season for the Premier League veteran.
Defoe told The Mirror that, should Southgate select him for the finals in Russia, he could provide the crucial spark for the Three Lions.
"I'd come on and win the World Cup, it's as simple as that," the 35-year-old said. "You come on for one minute, two minutes and if there's a chance, you score. At a tournament it's not just about the players who start, it's about the squad."
Speaking of his equaliser against the Hornets, the forward said: "I can't remember who was next to me but I knew he was the last man and I knew Nathan Ake was going to win the header.
"I gambled and it happened quickly but I knew I had to relax and take my time. I was going to hit it first time but you want to make it as easy for yourself as possible so I let it bounce and finished it."
Defoe's late goal moved Bournemouth to 37 points, nine clear of the dropzone.