The couple attended the star studded SNL after Party at Tao in New York City this weekend.
Previously, Johansson had kept their relationship on the 'down-low', but now they have chosen to make it official.
Johansson, 32, and Jost, 35, were seen hopping into a car as they left the club.
The pair flew under the RadarOnline.com at the event, which was also attended by the likes of Beyonce and Ryan Gosling.
The actress looked glamorous in a white crop top and high waisted trousers, showing off her elaborate back tattoos.
Jost set tongues wagging when he attended the Emmys last month without a date – but shared a few kind words about his new lady.
"She's wonderful – she's working, so otherwise, she'd be here," he revealed.
The lovebirds have been spotted out and about in New York City several times during the last few months but now they are really going public.
The Avengers starlet finalized her divorce from Romain Dauriac last week, just over a year after they separated following two years of marriage.
Court documents submitted last month in Manhattan claim the duo also came to an agreement regarding the custody of their three-year-old daughter Rose Dorothy.
The actress was also married to Ryan Reynolds from 2008-2011.
We pay for juicy info! Do you have a story for RadarOnline.com? Email us at email@example.com, or call us at (866) ON-RADAR (667-2327) any time, day or night.