Stewart and his fiancee, model/photographer Penny Lancaster, confirmed Wednesday that she is pregnant. The baby is due in early December and the couple plan to marry in spring 2006, said a statement from J Records.
The 60-year-old rocker told People magazine that he is "overjoyed and extremely proud."
"The best news of all is Penny is feeling just wonderful, is very happy and looks more radiant than ever," Stewart told the magazine.
Lancaster, 34, and Stewart have been together for nearly six years. He proposed to her in March at the top of the Eiffel Tower in Paris.
This will be her first child, their statement said.
Lancaster's agent, Nicholas Young, has said the couple would make wedding plans once Stewart's divorce from model Rachel Hunter is finalized.
Stewart's children include Kimberly, 25, and Sean, 24, by first wife, Alana Stewart; Ruby, 17, with former girlfriend Kelly Emberg; and Renee, 13, and Liam, 10, from his marriage to Hunter.