Holmes will star as the former First Lady in a new miniseries for the History Channel.
Speaking to Access Hollywood, Holmes explained that she is pleased she landed the role.
"I'm really grateful for this role and I've always been a fan of Jacqueline Kennedy," she said. "I've learned so much from my research and it's such a pleasure to be a part of this."
Holmes added that wearing outfits which replicate Onassis's clothes has been "thrilling".
The Kennedys, which also stars Greg Kinnear and Barry Pepper, will air next year on the History Channel.