Lively appeared on the pink carpet at the AMC Lincoln Square Theatre on a drizzling New York City, reviving Spears' 2002 vintage Versace butterfly dress adorned with sequin flowers.
"The ultimate queen who made us all want to sparkle and write and share our stories," Lively said on her Instagram story, sharing a picture of Britney in the dress.
"Britney, us millennials all have a story of a moment, or of years that you made us want to shine and inspire awe, with strength, and joy and immensely hard work.
"Thank you for your example and your contribution to women telling their stories. So excited about your biopic and all you have to come," she said.
"So when this dress was available I was like yes, I need it and I've had it for almost a year now and I've been saving it for this."
It comes after it was reported Spears' 2023 hit memoir The Woman In Me, will be adapted for the big screen.
Lively appeared at the world premiere alongside her co-stars Justin Baldoni and Brandon Sklenar.
Based on the best-selling novel by Colleen Hoover, the film tells the story of Lily Bloom who falls in love with Ryle Kincaid before reconnecting with her first love Atlas Corrigan.
The premiere also saw a Deadpool & Wolverine reunion for stars Ryan Reynolds - who is married to Lively - and Hugh Jackman.
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