The couple announced in March that they will welcome their first child later this year, and Williams appeared to confirm rumours they are expecting a daughter during a press event ahead of this weekend's Soccer Aid fixture for UNICEF.
"It's a girl. I'm very, very excited about becoming a daddy. I can't wait," The Mirror quotes Williams as saying. "The missus is like, 'Don't go telling', and I said 'I won't'."
Williams also joked that he intends to teach his daughter "football, karate and boxing", adding: "That's the first thing she's going to learn to keep people like me and Johnny (best friend Jonathan Wilkes) away."
The Take That star's bandmates Mark Owen and Gary Barlow are also expecting additions to their families this year.
Owen is due to welcome his third child with wife Emma Ferguson this summer, while Barlow and his wife Dawn are also expecting their fourth child, a daughter, soon.
Soccer Aid 2012 takes place at Old Trafford Football Ground in Greater Manchester on Sunday, May 27. The match will air on ITV1 from 6pm.
Watch a clip of Robbie Williams and Jonathan Wilkes on a UNICEF visit to Mexico City below: