Taking to Instagram, she posed for a sweet photo which showed her holding her one-year-old daughter in the air while her baby bump was on display.
The Made in Chelsea star looked beautiful in a pale pink one-piece swimsuit as her long hair flowed freely.
The 31-year-old captioned the lovely image: "We are so excited to announce that Sienna is going to be a big Sister! I couldn’t keep this to myself for much longer and I’m running out of ways to hide my bump! Baby 2 is due later this year [heart emoji]."
WATCH: Millie Mackintosh's heartwarming baby Q&A
Millie's fans were clearly thrilled for her and rushed to the comment section to share their excitement. "Yayyyyyyyy," one wrote, while others added: "Awww congratulations Millie," and: "The best news."
Several posted heart emojis alongside their congratulations.
Millie shared the exciting news with fans on Instagram
The star's husband Hugo Taylor was among her excited followers and left his own sweet comment, which read: "Yay!! Happy happy happy days."
The couple have been married for three years and welcomed their daughter in May 2020.
Millie revealed that her name was Sienna and shared the first official photos of the new arrival in HELLO! magazine last June.
In the exclusive interview, the devoted mum revealed that Sienna was born via Caesarean as she was in breach position.
Millie and husband Hugo share daughter Sienna, one
"We were supported by such incredible nurses, doctors and midwives throughout the whole process," Millie said.
She went on to gush about her love for her firstborn, saying: "She is so adorable, even down to the little noises she makes."
She went on: "We've been on cloud nine; it's gone so quickly. We've been in our little love bubble at home, cherishing those newborn moments, whether that's her sleeping on me or just lapping up all the cuteness."
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