The Scottish singer and record producer was unimpressed when he listened to a sample of the star's work in January 2008.
GaGa has since secured international success with her debut album The Fame and its first two singles 'Just Dance' and 'Poker Face'.
Speaking to The Sun, Harris admitted: "I usually make a lot of bad decisions if I'm left to myself.
"A perfect example of that is when Lady GaGa first came out. Her name rang a bell, so I checked my emails and it turns out that on January 1, 2008, I got an email asking to work with her. I thought: 'Lady GaGa, what sort of name is that?' So I replied saying: 'Nah, I'm not into that'.
"Fast forward a year and she's the biggest thing. But I don't regret turning her down as I didn't think the song I heard then was that good."
Harris is expected to top the UK singles chart tomorrow with his latest offering 'I'm Not Alone'.