Tate - who was heavily pregnant - was murdered along with four others in the home she shared with Polanski, when four members of the infamous Manson family invaded.
Robbie's character plays a central role in Tarantino's movie, which takes place in Los Angeles in 1969, at the height of hippy Hollywood.
The image, posted to Instagram, shows Robbie wearing a black top, white mini-skirt and boots.
"First look. #OnceUponATimeInHollywood @onceinhollywood" Margot wrote, tagging the upcoming movie. "It's always been a bucket list thing for me to work on a Tarantino film so I'm obviously ridiculously excited and grateful", she said.
The movie follows former Western TV star Rick Dalton (DiCaprio) and his stunt double, Cliff Booth (Pitt), who are, according to the film's press release, "struggling to make it in a Hollywood they don't recognize anymore".
Her sister Debra originally accused the film, and another in the works starring Hilary Duff, of being "exploitative", due in part to their release dates being planned to coincide with the 50th anniversary of the murders.
One thing we do know is that the unbelievable cast will also include Al Pacino, Timothy Olyphant, Damian Lewis, Dakota Fanning, Emile Hirsch, Tim Roth, Michael Madsen, Scoot McNairy, Clifton Collins, Luke Perry, and Burt Reynolds.
As previously announced, Robbie is playing Sharon Tate, the most famous victim of the Manson murders.
Do you remember the horrific murder of Sharon Tate?
The film will hit theaters on July 26.