The 38-year-old Percy Jackson and White Lotus star shared a photo of the child wrapped in a blanket with various chocolate wrappers on top of the baby.
Next to the Instagram post shared on Halloween, she wrote: “I thought it was a weird bowl of candy.”
Emily in Paris star Lily Collins was among those to send their congratulations to the actress, writing: “Oh congrats!!!!!!!!!”
Daddario announced she was expecting a baby with husband Andrew Form, 52, in an interview with British Vogue earlier this year.
The actress and her film producer husband got engaged in 2021 and married in June 2022.
She became famous for her role as Annabeth Chase in the Percy Jackson film series and has appeared in a number of films and shows.
Her role in the first series of the hit drama The White Lotus earned her Emmy nominations for Outstanding Supporting Actress in a Limited or Anthology Series or Movie.