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.
Duggar sisters unveil Christian dating rules in new book
Astronomers offer more expansive view of universe