The 33-year-old actress and 35-year-old comedian attended the annual event Monday at the Metropolitan Museum of Art Costume Institute in New York.
Johansson wore a red Marchesa dress with an off-shoulder neckline and floral embellishment. She held onto Jost's arm as they posed for photos on the red carpet.
Johansson is the first celebrity to wear Marchesa, a brand owned by Harvey Weinstein's ex-wife Georgina Chapman, to a high-profile event since Weinstein's sexual harassment and abuse scandal last year.
Chapman split from the producer in October.
"My heart breaks for all the women who have suffered tremendous pain because of these unforgivable actions," she told People at the time.
Johansson and Jost were first linked in May 2017, and made their red carpet debut as a couple at the Los Angeles premiere of "Avengers: Infinity War" in April.
"Their eyes lit up when they looked at each other," a source told Us Weekly.
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Jost, who hosts the "Saturday Night Live" segment "Weekend Update," referred to Johansson as his girlfriend during a sketch with Leslie Jones on Saturday's episode, saying, "I told you I have a girlfriend."
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