The Factory Girl actress and partner Oli Green welcomed their second child earlier in 2026 and now it has been revealed they are getting married.
The 44-year-old star - who also has daughter Marlowe, 13, with former partner Tom Sturridge, and a two-year-old daughter with Green - stepped out in Spain last week sporting a huge diamond ring on her engagement finger and E! News confirmed the sparkler is an engagement ring.
The couple are largely private about their lives, keeping their kids out of the public eye, and it only emerged Miller had given birth to her third child when she let the news slip last month while promoting her latest project, Jack Ryan: Ghost War.
"It's happened. I have a tiny baby next door," she told E! News.
"It feels like stringing sentences together is a bit challenging.
"I'm on very little sleep but I'm madly in love with my baby."
The American Sniper star recently admitted adjusting to her expanded family hadn't been easy.
"I would have said the teenage girl would be harder to juggle until the transatlantic flight I took yesterday with the toddler and the newborn," she told The Tonight Show in May.
"The toddler now wins hands down. It was an absolute disaster. There's no negotiating.
"The looks people give you on the flight. The baby's screaming, she's screaming. It was like, 'Yeah.' I got to the immigration line and just broke down."
Miller - who has been dating Green for four years - recently admitted she has found being pregnant in her 40s "so much easier" than her 20s because her life is in a "more grounded space" now.
"Having had a baby at 29, and then having a baby at 42, and now 44, it's so much easier when you don't have the conflict of feeling scattered and like you want to be doing X, Y, Z," she told Glamour magazine.
"If I'm in bed at 9pm with a book, I'm so happy now.
"And now I've got the excuse to do it.
"Life is in a more grounded space. I think the 30s are chaos."
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