We now know who Katy Perry's 'I Kissed a Girl' is about. And we did not see this coming.

Almost nine years after Katy Perry told the world that she kissed a girl and liked it, we’ve found out who the song is about.

Only the revelation has us raising our eyebrows.

Turns out, according to the woman herself, Katy’s breakout 2008 hit I Kissed a Girl is about Miley Cyrus.

The Malibu singer outed herself while gushing about the pair’s nine-year friendship on a US radio show with KTU’s Cubby and Carolina in New York.

“We were actually just realising the other day that next year, we’ll have been friends for 10 years,” Miley told the hosts on Tuesday.

“I think that’s my friend that I’ve known the longest. Which is really, really weird.”

But it’s about to get weirder. Because Miley, 24, explained that Katy’s song was written about her – even though their friendship began after the single’s release.

"When [Perry] came out with I Kissed a Girl, I was doing the Hannah Montana movie, and I heard her on the radio."

"They said, ‘Who did you write that about?’ She said me! I screamed and started freaking out, and then she asked me to go to the VMAs with her.

"That’s when I started doing my whole VMA controversy. I was like, ‘I’ll go with Katy Perry!’ So that’s how we met and we just stayed friends."

Listen: Chrissy Teigen knows all about red carpet controversy.

So Miley was the subject of the song, the inspiration perhaps, and Katy told the world in 2008 - and we all missed it?

Well, it would make a whole lot more sense than a then 23-year-old Katy kissing a then 15-year-old Miley. (Yep, there's an eight-year age gap.)

However, we still have more questions. Such as, what was it about a Hannah Montana-era Miley that made Katy want to kiss her? Or was she just picking up vibes that the teenage Disney star would be open to experimentation and putting herself in her shoes?

Either way, it seems we have Miley Cyrus to thank for Katy Perry's first number one song, and arguably her entire career.

What's your favourite naughties pop song?