Grande posted a selfie of the two pop stars back in August on Instagram and in it, Bieber can be seen giving Grande a smack on the cheek.
The "Baby I" singer said the two got the idea to take the photograph because fans were asking for one online.
"I think [fans] had trended #JustinAndArianaSelfie or something like that, so I was like, 'we have to do this.' Justin was like 'Yo, let's take a picture,'" Grande said. "And so I took out my phone and I went like this and I was just smiling and he kissed me on the cheek."
She said she became nervous after the kiss.
"And I didn't know what to do. I just walked out of the room. I got shy," Grande said. "And then [Justin's manager] Scooter came after to me and said, 'Justin says you have to post that picture.' "
Grande said she didn't want to post the picture because of the drama she expected from it, but did so anyway. The photo has since been removed from her Instagram account.
She took to Twitter to defend herself after backlash over the photo from fans of Bieber's on-again, off-again relationship with fellow musician Selena Gomez.
Everyone, girls are allowed to be friends with guys. I'm a lady. Chill out.— Ariana Grande (@ArianaGrande) August 9, 2013
Grande said despite being on tour with Bieber over the summer, the two hardly ever saw one another.
"It's funny, everybody thinks we spent so much time together because we were touring, but really, I think I saw him once, and it was for about five minutes," she said. "He played me 'Heartbreaker,' it was amazing, and then I left. It's like 'Hey, great job, bye.' "