The 42-year-old singer - who recently split from her husband Josh Duhamel - previously romanced the 'Can't Stop The Feeling' hitmaker, and whilst she insists the fling wasn't a serious relationship, she refused to comment further.
Fergie was taking part in 'Plead The Fifth' as part of her appearance on 'Watch What Happens Live with Andy Cohen' when the show host asked her about her fling with Justin - who is now married to Jessica Biel, with whom he has two-year-old son Silas.
She said: ''It wasn't all that serious. Am I allowed to pass on this? Pass. Plead the fifth.''
It isn't the first time the former Black Eyed Peas member has been asked about her romance with Justin though, as in 2006 she admitted they would ''have fun and make out'' whenever they spent time with one another.
She said at the time: ''One of my good friends was dating J.C. Chasez, and so we'd all hang out with 'N Sync. Justin and I would go out together and have fun and make out. We went to Hawaii together, but we were never serious. He was Mr. Right Now.''
Meanwhile, the 'Big Girls Don't Cry' singer - who has four-year-old son Axl with her estranged husband - admitted she isn't looking to start dating just yet, following her split earlier this year.
She said: ''I don't know. It's too soon for me to even think about dating. I'm open to love, but it's just too soon.''
Fergie previously admitted she still has ''so much love'' for Josh, and she and the 44-year-old actor will always be ''great friends''.
She shared: ''We're great friends, Josh and I, there's so much love that we have for each other. We really like each other, we have fun with each other. We're just not a romantic couple anymore.
''But we are still Axl's parents through and through, and our families - each other's families, the cousins, my nieces and nephews - it's all the same. So that's the only thing that's changed.''
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