Kieran Culkin, Scarlett Johansson Are Not Dating

Credit: KCSPresse/Splash News Online

Home alone -- and abroad -- with Macauley Culkin's little bro?

Although Scarlett Johansson had a super-flirty all-nighter with Justin Timberlake Sept. 14, he's not the only Hollywood guy she's hanging out with.

On Sept. 12, Johansson, 26, was photographed strolling in Paris with actor Kieran Culkin. Five days later, she and Culkin, 28, were seen strolling arm-in-arm outside Manhattan lounge Cabin Down Below.

But a source tells Us Weekly that Johansson and the Scott Pilgrim vs. the World star, 28, are just old friends who went to high school together.

"She has never hooked up with Kieran," the source insists.

After she and Ryan Reynolds finalized their divorce earlier this year, Johansson dove into a relationship with Sean Penn; the 50-year-old Oscar-winning actor ended the romance after five months in late May.

She and Timberlake, meanwhile, have "hooked up several times over the years," a pal explains. But despite their chemistry, "they've always been fine keeping it just sex and friendship."