The couple has been together since July, UsMagazine.com noted. Van Der Beek and actress Heather McComb reportedly separated in June after nearly six years of marriage.
"Yes, it's true -- the woman I love and I are expecting our first child," the actor Tweeted about Brook's pregnancy Friday. "Currently experiencing joy on levels I never knew existed."
The actor did not specify the baby's gender or when Brook is due to deliver.
Van Der Beek is best known for his work on TV's "Dawson's Creek" and "Mercy."