The couple made the surprise announcement on Valentine's Day, by sharing a picture on Instagram featuring the feet of all their four children.
Ayda said their son, named Beau Benedict Enthoven Williams, was born via the same "incredible" surrogate as their third child, Coco.
She added that the latest addition made their six-person family "officially complete."
She wrote: ‘‘Spot the difference…
"On this Valentine’s Day, we would like to celebrate love in the most awesome way… Beau Benedict Enthoven Williams."
The arrival of Coco - whose name is Colette - was announced in September 2018.
Ayda, 40, and Robbie, 46, have two other children - daughter Theodora, know as Teddy, and son Charlton, known as Charlie.
Ayda continued: "As with Coco, he is biologically ours, but born via our same incredible surrogate.
"We are so blessed to have our healthy son safely in our arms and are officially complete as a family."
She added the hashtags #happyvalentinesday #surprise #familyof6 #whodathunkitinthe90s.
When the couple announced the arrival of Coco, Ayda said she had endured a "long and difficult path" to have a third child.
She wrote at the time: "So we have been keeping a very special secret.
"We are delighted to share with you that we have had a baby girl…. welcome to the world Colette (Coco) Josephine Williams!!
"It has been a very long and difficult path to get here, which is why we have kept it on the down low.
"Family comes in all forms, and this little lady, who is biologically ours, was carried by an incredible surrogate mother, to whom we will be eternally grateful."
Last year, Ayda said she was keen to have a fourth child.
Speaking on the Loose Women panel in October, she said: "I'd like to, [but] you never know what's going to happen..
"I'd like one more, but after that I think Rob would commit me somewhere and run away with Gary Barlow into the sunset."
She added: "Being a mum is my greatest pleasure."