The 28-year-old actress and her team sneaked up to the iconic 14-metre Hollywood Hills landmark with bags full of lingerie from her new SYRN brand, having scaled a fence and climbed up the hill.
In a video shared on Instagram, they then hung lines of bras over the famous letters, and posed in front of the sign afterwards, before removing the lingerie - although it's alleged some were left on the hillside.
Although they reportedly had a filming permit, they could still be in legal trouble over their actions.
The Hollywood Chamber of Commerce - who own the IP rights to the Hollywood sign's image - have revealed a "license or permission" is needed to access the landmark, which is managed by the Hollywood Sign Trust.
The chamber's president and CEO Steve Nissen told ABC News in a statement: "The production involving Sydney Sweeney and the Hollywood Sign was not authorised by the Hollywood Chamber of Commerce nor did we have prior knowledge of it.
"The Hollywood Chamber of Commerce did not grant a license or permission of any kind to the production involving Sydney Sweeney as reported by TMZ, nor did anyone seek a license or permission from the Chamber for that production."
According to TMZ, the team had permission from FilmLA to shoot on location, but it's claimed they didn't have approval to climb, touch or physically alter the sign.
Although the sign is on public land, it's in a restricted area, and there are laws against climbing the structure.
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