The 21 year old star - recently voted the sexiest woman alive by Esquire magazine - admitted she has a HIV test twice a year.
Johansson, admired for her hour glass figure, said: "There does seem to be a mistaken belief out there that I am sexually available somehow which is not to say that I'm not open-minded about sex.
Contrary to popular belief, I'm not promiscuous. "Yet I wouldn't say I'm a serial monogamist, either. I mean, I went through periods of time when I was, ah, single. But when I'm in a relationship, I'm in a relationship."
The actress said she was happy in her current relationship with actor Josh Hartnett, adding:" He's a good boy. A great person. I'm very lucky and I'm very happy."
In an interview with US magazine Allure, Johansson agrees with fellow actress Sienna Miller that when it comes to sex humans are very much like animals. Miller said last week that monogamy was a "weird virtue".
She said: "It's an overrated virtue, because, let's face it, we're f***ing animals."
Johansson was less graphic in her terms but told Allure magazine:"I do think on some basic level we are animals, and by instinct we kind of breed accordingly.
"But as much as I believe that, I work really hard when I'm in a relationship to make it work in a monogamous way."
Johansson and her boyfriend Hartnett met while filming "The Black Dahlia", the story of an infamous murder in 1940s Los Angeles.
The actress has been linked to a string of A-list stars, including Jared Leto.
She was also forced to deny having sex with actor Benicio Del Toro in a Los Angeles hotel lift after making a sarcastic remark to a journalist.
The actress says she undergoes AIDS/HIV tests twice a year and said people who fail to take the necessary health precautions are irresponsible.
"I get tested for HIV twice a year. One has to be socially aware," she said.
"It's part of being a decent human, to be tested for STDs. It's just disgusting behavior when people don't. It's so irresponsible."
