The delighted singer - who is also mum to 10-year-old daughter Bluebell, added a baby and bottle emoji to her tweet.
She then took to Instagram to share a snap of her holding her son's tiny foot, announcing his name with: "Montague George Hector Horner arrived this morning, a beautiful little brother for Bluebell and Olivia #amazing-day #grateful #monty".
The birth coincides with Geri's ex bandmate and gal pal Emma Bunton's birthday, with Emma taking to Twitter to congratulate her with the hashtag 'birthdaytwins'.
Geri's first Saturday tweet was a birthday message to Emma. She shared a throwback of the duo in their Spice Girl days and a heart emoji before sharing her baby news.
"Amazing news, so happy! #Birthdaytwins #Birthdaymade love you all. Xxx", Emma replied happily, and fellow Spice Girl Melanie Chisholm also tweeted her congratulations.
And the singer's fans were delighted with the news, with her tweet garnering almost 1000 likes in just twenty-five minutes.
The news comes shortly after it was reported Geri had dropped out of the Spice Girls reunion 'GEM'- where she was due to unite with Mel B and Emma, to focus on her family.
A source told The Sun before her birth: "Geri's priorities have changed. Her baby is due later this year and rather than force a reunion, she's needs to concentrate on more personal matters.
"Getting all the girls in the studio at the same time proved a tough task. What they needed was one main boss-style figure to pull the strings otherwise they would be constantly in limbo."
