Defoe, who hails from Stratford in east London, asked for a transfer request after the Hammers were relegated in 2003 and moved to Tottenham.
The 31-year-old is now in his second spell at White Hart Lane, but he insists he still regrets his decision to walk out on West Ham.
He told ITV4: “It was a massive mistake. I didn’t really want to do it.
“We’re talking about leaving the club I was in love with at the time because I came through the youth system with my friends, the fans loved me there. They were brilliant - every game they sang my name.
“The person who represented me at the time said, ‘You need to hand in a transfer request and get in early because all the lads are going to leave.’
“I remember going in and I was so nervous. It was (then Hammers chief executive) Paul Aldridge and I gave him the letter and he said, ‘What’s this?’ And I said, ‘It’s a transfer request’.
“And he looked at me as if to say, ‘Are you kidding?’
“The backlash was crazy. I felt like I was on my own and people were absolutely abusing me.
“To this day I can imagine West Ham fans look at me and think, ‘Who does he think he is? How can you do that? You’re a Judas’. And it’s quite sad really because I can honestly say I still love that club.”