Johansson, 22, stars in Timberlake's new video for "What Goes Around" as, ironically, a femme fatale, the celebrity gossip magazine reported this week.
US Weekly quoted "a Timberlake pal" as saying that "Cameron was very jealous of Scarlett ... She could not believe he went behind her back to put her in the video."
The source told the magazine that Johansson and Timberlake "definitely got together after she did the video."
Other unnamed snitches from both Timberlake's and Diaz's "inner circles" told the magazine that Britney Spears' ex-boyfriend and Johansson have just embarked on their relationship and are looking forward to getting to know each other better.
A friend of Johansson's said the actress — who starred in "Match Point" and "Lost in Translation," among other films — has had a longtime thing for Timberlake, according to US Weekly.
"Scarlett has always liked Justin," the unidentified source said. "They've always had a flirtation — that's why Cameron was threatened by her."
After months of fending off rumors of their split, Timberlake, 25, and Diaz, 34, confirmed Thursday in a joint statement that they had broken up.
"We have, in fact, ended our romantic relationship, and have done so mutually and as friends, with continued love and respect for one another," said the pair, who have been dating since 2003, after meeting at the Nickelodeon Kids Choice Awards.
Diaz and Timberlake said that in the past they've shied away from discussing their relationship, but they felt it necessary to do so this time in order to set the record straight.
"It has always been our preference not to comment on the status of our relationship," their statement said, "but, out of respect for the time we've spent together, we feel compelled to do so now, in light of recent speculation and the number of inaccurate stories that are being reported by the media."
The couple broke up before the New Year, US reported.
Timberlake just launched his "FutureSex/LoveSounds" tour and his feature film, "Alpha Dog," is scheduled to open this weekend. Diaz most recently appeared in the romantic comedy "The Holiday" with Kate Winslet.
Timberlake and Diaz are "starring" together in the forthcoming animated movie "Shrek 3."
FOX News' Catherine Donaldson-Evans and The Associated Press contributed to this report.