The 33-year-old actress and 35-year-old writer and comedian attended the Los Angeles premiere of Johansson's movie Avengers: Infinity War at El Capitan Theatre in Hollywood.
Johansson turned heads in a metallic, embellished dress with a belted waist and sweetheart neckline. The star was all smiles as she posed for photos with Jost, a writer and performer on Saturday Night Live.
"Their eyes lit up when they looked at each other," a source told Us Weekly.
Johansson plays Black Widow in Avengers: Infinity War, which opens in theaters Friday, April 27. The movie co-stars Robert Downey, Jr., Chris Hemsworth and Mark Ruffalo, who also attended the premiere.
Johansson and Jost were first linked in May 2017 following Johansson's appearance on SNL. The actress discussed Jost on Tuesday's episode of The Ellen DeGeneres Show, saying the comedian's "Weekend Update" segments are her favorite SNL sketches.
"I guess I'm a Colin fan," she gushed.
Johansson split from Romain Dauriac in 2016 after two years of marriage, and shares 4-year-old daughter Rose Dorothy with her ex-husband. She will also star in the film Jojo Rabbit from Thor: Ragnarok director Taika Waititi.