Credit: Stephane Cardinale/People Avenue/Corbis; Alex Berliner / BEImages

Jessica Biel's face will go scarlet when she hears about this!

The new Us Weekly reveals that Justin Timberlake spent the night of Sept. 14 with another Hollywood beauty: Single-again Scarlett Johansson, his smoking-hot costar in his 2007 "What Goes Around Comes Around" video.

Timberlake, 30, and Johansson, 26, were "hardcore flirting" during a raucous evening at NYC hotspot 1Oak, a witness tells Us. "Other guys were trying to talk with Scarlett, but she was only focused on Justin."

The duo literally partied until dawn --leaving at 5:30 a.m. and getting pizza next door, the source says. The one-on-one party continued just a few hours after the A-listers closed down 1Oak -- when Timberlake showed up at the Iron Man 2 actress' midtown penthhouse apartment.

Greeting the doorman and introducing himself as "Justin," Timberlake "wasn't even low-key" about his visit, another witness noted. (Reps for both stars deny the visit took place.)

"They've hooked up several times over the years," a pal confirms of the pair. But Timberlake just got back together with longtime love Biel, 29, merely a month ago.

"For the moment," another insider says, "she's trusting him."

For more on Timberlake's latest betrayal -- and what's going on with Johansson and Kieran Culkin -- pick up the new Us Weekly, out now!