Fans first spotted the new tattoo when she posed in skimpy shorts in the August issue of Vanity Fair.
"Ohh, it's for Brad, it is something for Brad," Usmagazine.com quoted her as saying when asked about the tattoo by MTV News.
The actress, who has been with Pitt, 45, for five years, was more open when asked what makes a relationship successful.
She added: "I actually think it's important to be completely honest, however difficult that may be, I'm bad at hiding secrets anyway." (ANI)