The pair have gotten engaged after two years of dating, the actress' rep confirmed to ET on Sunday, adding, "No date has been set for the nuptials."
Johansson, 34, has been married twice before -- first to Ryan Reynolds before their divorce in 2010, and then to French journalist and advertising exec Romain Dauriac, with whom she shares a 5-year-old daughter, Rose. The couple later divorced in mid-2017 after separating a year earlier.
Meanwhile, this will be the first marriage for Jost, 36, who sparked a romantic relationship with Johansson -- a recurring SNL guest host -- in May 2017.
Most recently, the couple were spotted walking arm-in-arm on the red carpet at the Hollywood premiere of Avengers: Endgame in April.
ET spoke with Johansson at the star-studded event, and she joked about her plans to use Jost to play a prank on her Avengers co-star, Paul Rudd, when he hosted the season finale.
"I've got connections. I'm gonna make sure that his time is really difficult, you guys," Johansson explained of her plan, which she seemingly didn't actually enact when Rudd took the stage on Saturday. "I'm gonna sabotage him. I'm gonna come in from the inside [so] he doesn't know what's coming."
For more on Johansson and Jost's adorable romance, check out the video below.