The actress, along with her boyfriend Jason Walsh, dressed up as a pilgrim and Native American, respectively. The two were attending the annual Casamigos Halloween party in Los Angeles Friday.
"I am SO sorry to people I offended with my costume. It was not properly thought through and I am truly, from the bottom of my sorry," she tweeted Sunday.
Walsh also offered an apology.
Native American costumes have long drawn the ire of indigenous people, who believe their culture is being appropriated by others.