Preston and Travolta are already the parents of a 10-year-old daughter, Ella Bleu. Their teenage son, Jett, died last year of a seizure disorder.
"It's impossible to keep a secret ... especially one as wonderful as this," Preston and Travolta said in a statement. "We want to be the first to share this great news with everyone that we are expecting a new addition to our family. Love, John, Kelly and Ella."
The online message did not mention the baby's gender or due date.