The England striker scored his first Bournemouth goal in 16 years as Eddie Howe's men came from behind to win their first Premier League game of the season.
Solly March's 55th-minute header had Brighton dreaming of a second successive league win but Andrew Surman coolly levelled before Defoe stole the victory - with substitute Jordon Ibe setting up both home goals.
After the game, Defoe told Sky Sports: "I thought when we went a goal down, we really dug in - the tempo changed completely. We were on the front foot. Credit to the lads.
"On a personal note, I'm just delighted to get my first goal back and it was the winner so we're delighted with the three points."
Having lost their first four games of the season, Defoe said it was great for the club to get off the mark at the fifth attempt but warned that Bournemouth need to continue to improve.
"I feel like we’ve still got more to give and I’m sure the manager will say the same because each week we’re improving," he told the club website.
"I felt like we controlled the game tonight, we created chances and had good energy levels. The character of the players was excellent tonight and the attitude was equal to that.
"Although, we don’t want to take our foot off the gas. We have to regroup and focus on the next game."