"Honestly, it brought a closure to me that I needed," the actress said on Monday after mentioning that she finished production on the Marvel film in London a couple weeks ago.
"I was kind of, you know, wiped out after that last one. I mean emotionally and literally," Johansson continued in reference to Avengers: Endgame, where Black Widow met her untimely end.
Johansson also commented on her character's status following Endgame and how fans don't want to believe that the talented spy is really gone.
"There's very few certain things, but like death's pretty certain right?" Johansson said. "I think death is a pretty final thing."
The Black Widow film, from director Cate Shortland, will take place in the past. David Harbour also stars.
Johansson also told Fallon how she doesn't want her fiance Colin Jost's bachelor party to be hosted by Jost's Saturday Night Live partner Michael Che. Johansson and Jost got engaged in May.