The pop singer took to Twitter.com on Monday (24Aug15) to announce the happy news, revealing the couple's first child together will arrive early next year (16).
He writes, "I have some amazing news to share with you all... Katie and I are having a baby! so proud of my beautiful wife and can't wait for the newest member of our family to arrive."
He added, "Thank you for all your wonderful messages. The baby is due early jan (January). Boy or girl? Guesses on a postcard plse (sic)."
Donald and illustrator Halil exchanged vows in January (15).
The singer has two daughters, Grace, 16, and Lola, 10, from previous relationships.