Williams revealed the good news on his website on Friday.
He wrote: “Hey friendlies, I’ve been keeping a secret from you all. Me and Ayda are going to be a Mummy and Daddy this year!!!!!!!!”
“We had sex! It works! We’ve seen scans and cried… looked at babies in commercials and teared up… the nursery is already planned,” he added.
“Basically I’m in love with a little person growing in mummy’s belly… and can’t wait to be a daddy. Crying now. Much love, much joy. Roberto, Mum and Little Bump. Xxx”
The Daily Star Sunday wrote that the singer is ecstatic over discovering the sex of his first child, which is due in September.
“He’s always wanted to be a dad so he’s going all out so his new daughter will be treated like a princess. He wants her to have the best of everything,” a source told the newspaper.
In addition, Robbie and Ayda are reportedly already discussing baby names, with Genevieve, Alice and Roberta making the list of names.
Robbie Williams, 38, married Ayda Field, 32, in August 2010.