"Olivia Marie Armstrong has arrived!" he wrote early Monday morning, keeping mum on the specifics.
"Thanks to you all for the support and well wishes. Momma and Olivia (@Cincoarmstrong!) are doing great. She's a beauty!"
This is the Tour de France champ's second child with Anna Hansen, with whom he also has son Maxwell Edward, 16 months. Additionally, Armstrong is dad to twin daughters Isabelle and Grace, 9 next month, and son Luke, 11, with ex-wife Kristin Armstrong.
Armstrong, 39, announced the pregnancy in April.