"What a beautiful day to be born..." she wrote. "Welcome to the world Buddy Bob Fletcher. xxx #player2."
Just minutes earlier her husband Tom, 30, announced the birth, writing: "Player 2 has entered the game."
Giovanna Fletcher has welcomed a second son
The McBusted singer followed it up with another tweet, posting: "Completely overwhelmed with love for our second son Buddy, and can't wait for him to meet his big brother. Thanks for the lovely messages. X."
The couple were quick to receive congratulatory messages on the social networking site. Tom's proud mum Debbie quickly expressed her joy at the news, tweeting: "What a day for the Fletcher family."
She added: "Massive congratulations to @tommcfly and @MrsGiFletcher and thank you for my new grandchild x."
The Fletchers' love story dates back 12 years – a milestone they celebrated by sharing throwback photos on Instagram. The duo married in 2012, but they've known each other a lot longer after meeting at the Sylvia Young Theatre school in London.
Tom and Giovanna, 31, announced the happy news of her second pregnancy with an incredible video in September followed by pictures of her having a scan. The author kept fans updated on her progress throughout, including sharing how first-born son Buzz was feeling. She said her toddler was excited that a new sibling was on the way.
Tom and Giovanna are already parents to son Buzz
The doting mother worked right up until the last moment of her pregnancy, as she wrote in her HELLO! Online blog: "The lead up to Christmas is scheduled to be fairly manic with writing my fourth novel, promoting Dream a Little Christmas Dream and filming for Lorraine. Oh wait, did I mention vlogging, blogging, being a wife, mother to a toddler and getting ready for our next bambino to arrive?"