The 27-year-old actress approached the Black Hawk Down hunk while celebrating the start of New York Fashion Week at the DeLeon Tequila party at Rose Bar.
"She arrived with two men and sat at a table right in front of the stage. In the middle of rocking out to the music that was playing, she eyed Josh and instantly leapt out of her seat to shower him with a public display of affection," Contactmusic quoted a source as having told Fox News.
The 31-year-old actor was apparently quite flattered with the attention, and quickly returned the G.I. Joe star's advances. But the open flirting did not quite impress Sienna's pals, who failed in persuading her to leave and eventually left her to her own.
Goth rockers The Cult gave a special performance at the party, which was attended by the likes of Hugh Grant, socialite Nicky Hilton and supermodels Helena Christensen, Alek Wek, and Bar Refaeli.