The Austin Powers star, 50, and Tisdale, 36, are already parents to 2-year-old son Spike.
"We are thrilled," the couple tell PEOPLE.
After growing up with a great father, Myers recently admitted he has always looked forward to starting a family.
"Anyone who tells you fatherhood is the greatest thing that can happen to you, they are understating it. I am the happiest I have ever been in my life," he told Deadline while promoting his directorial debut, Supermensch: The Legend Of Shep Gordon.
Plus: Having a boy or a girl? Check out the Ancient Chinese Gender Predictor!