In a post on Facebook on Friday night the premier said he had "exciting family news".
"Our family photo wall is going to need another frame! Helen and I are thrilled to announce we have a little girl due next year.
"With five sisters already, William is looking on the bright side - more bench depth on the family basketball team."
Mr Perrottet was appointed premier on October 5 after Gladys Berejiklian stood down to face a corruption inquiry.