Sienna Miller Brings Daughter Marlowe, 11, to “Horizon” Premiere at Cannes Film Festival

The actress' boyfriend, Oli Green, accompanied the pair on the red carpet

Sienna Miller had a special fan by her side at the 2024 Cannes Film Festival!

The actress, 42, attended the world premiere of Horizon: An American Saga with her daughter Marlowe, 11. She was also joined by her boyfriend, Oli Green.

For the big event, the actress wore a semi-sheer dusty blue gown with a ruffled cape. She paired the formal attire with strappy silver heels.

Miller embraced her daughter — who was dressed in an adorable pink and white A-line dress — as they posed for photos together.

The American Sniper actress shares Marlowe with her ex Tom Sturridge, whom she dated from 2011 to 2015. She also has a daughter, born in January, with Green, 27.

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Miller and Green have been romantically linked since February 2022 and made their red carpet debut at the Vanity Fair Oscar party the following month.

Related: Sienna Miller and Oli Green Have Rare Date Night Out at Anna Wintour's Pre-Met Gala Dinner

In December 2023, the actress opened up about the pair's 15-year age difference in a cover story for Vogue, saying, "I would imagine it would be complicated for anyone to get their head around, but there’s been nothing but love and joy."

Miller's upcoming movie Horizon is directed and co-written by Kevin Costner, and "chronicles the incredible epic journey of the expansion of the American West, before and after the Civil War," according to a synopsis.

Related: Selena Gomez Looks Gorgeous in Red During Emilia Pérez Photocall at Cannes — See Her Bold Look!

"Between the Native Americans who saw their lands getting colonized and those who were determined to settle there, sometimes at any cost, history is being written," the synopsis continues. "In a flamboyant fresco where multiple destinies intertwine, dreams and hopes face obstacles and cruelty to offer a cinematic spectacle of exceptional scope and emotional depth."

Miller plays maternal pioneer woman Frances Kittredge, who was brought to the Horizon settlement by her husband.

The Anatomy of a Scandal actress is joined by two-time Academy Award winner Costner, Sam Worthington, Jamie Campbell Bower, Isabelle Fuhrman, Luke Wilson and more in the cast.

Costner, 69, attended the event accompanied by five of his seven children on the red carpet: Annie, 40, Cayden, 17, Grace, 13, Lily, 37, and Hayes, 15 — the latter of whom appears in Horizon alongside his father.

Ahead of the premiere, Costner told PEOPLE that he was “taking seven of the women that are in the movie” to Cannes to celebrate their “huge contribution” to the project.

"Women are the reason the country goes forward," he explained. "They were taken often against their own wishes. They were taken advantage of and they were resilient. I couldn't tell the story unless it revolved around them."

Related: Kevin Costner Moved to Tears by 10-Minute Standing Ovation for Passion Project Horizon in Cannes: 'I’ll Never Forget This'

In the same interview, the Yellowstone star revealed what drew him to casting Miller in the movie, calling her “one of our great actresses.”

He said: “Her outward beauty is undeniable, but she's so skilled as an actress."

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Horizon: An American Saga Chapter One and Chapter Two are set to open in theaters on June 28 and Aug. 16, respectively.

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