The 37-year-old actress, who married Timberlake in October 2012, discussed the costume during Monday's episode of The Late Late Show with James Corden.
Biel said it was "fun" to dress as 'N Sync-era Timberlake for a Halloween party in October. Timberlake was a founding member of the boy band 'N Sync, which recorded and performed together from 1996 to 2002.
"This was his idea. This was all his idea. He said, 'You're going to do this,'" Biel said. "I was working in New York, so I was literally flying back on Halloween night. I had missed my flight and the whole crew of all our friends being the band members were waiting on me to get there."
Biel sported a curly blonde wig and a puffy blue jumpsuit that Timberlake actually wore during his 'N Sync days.
"I surprised Justin because we had that exact jumpsuit that he wore in storage. So we pulled it out and it was the real one," Biel said. "It's the real jumpsuit."
Timberlake, meanwhile, dressed up as a microphone for Halloween.
"It was the hugest costume," Biel said. "It was so funny. He could barely move all night because of this huge thing on his head."
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Biel had shared a photo of herself and Timberlake in costume on Instagram. In the caption, she referenced a recent interview where she said she didn't listen to 'N Sync during the group's heyday.
"This is what happens when you admit on TV that you don't know any NSYNC songs and you're married to @justintimberlake. Well played, husband, well played," she wrote.
Biel stars in a Facebook Watch series adaptation of the podcast Limetown. The show premiered in October and co-stars Stanley Tucci.
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