The Avengers star’s publicist Marcel Pariseau confirmed the happy news to the Associated Press on Sunday.
He revealed that the couple are yet to set a date for the wedding.
Johansson and Jost were first linked in 2017, after the actress appeared on the much-loved weekend comedy show.
They made their official debut at the American Museum of Natural History Gala in 2017, and have since attended red carpet events for Avengers: Infinity War and Avengers: Endgame together.
People magazine reported that wedding bells were on the horizon back in March, with a source telling the publication: “Scarlett and Colin are in love and share many of the same interests and the same sense of humour. Scarlett is very happy.
“There is marriage talk with Colin.”
The pair rarely discuss their relationship with the press, Jost described his partner as “wonderful” in a red carpet interview at the 2017 Emmy Awards.
“She’s pretty cool. It’s hard to have a lot of complaints, she’s pretty awesome,” he said, adding: “I’m very happy. I feel very lucky.”
The actress was previously married to Deadpool star Ryan Reynolds and to journalist Romain Dauriac, with whom she shares five-year-old daughter Rose.
Johansson most recently reprised her role as Black Widow in Avengers: Endgame and is set to appear in the WWII-set drama Jojo Rabbit later this year.
Jost is the co-host of Saturday Night Live’s Weekend Update with Michael Che and is a writer for the topical comedy show.