The couple announced the big news in a TLC video as well as Tori's Instagram account.
"We have some really exciting news to share with all of you," Zach notes in the clip before wife Tori excitedly exclaims, "We're having a baby!"
Zach, who is a dwarf, and Tori are anticipating their first child's arrival in May 2017. The newborn will be the first grandchild for Zach's parents, Matt Roloff and Amy Roloff, who also star on Little People, Big World. The family runs Roloff Farm in Oregon.
Tori shared ultrasound images of her pregnancy on Instagram, writing, "Zach and I are SO excited to FINALLY tell you guys the news! Thank you so much already for the love and support we have received! This is going to be one fun adventure! #ZandTpartyofthree."
The couple has yet to find out the sex of their baby and revealed to People they are "waiting until after Christmas" because they are "just enjoying the moment right now."
Zach admitted, however, he's "super curious" as to whether they are going to welcome "a dwarf baby or not." After all, Zach's three siblings are all of average height.
Zach, 26, revealed in the TLC video how his wife of 15 months first told him they have a baby on the way.
"I walked into the house and she had written it on a notepad that she was practicing her calligraphy, so I was reading through this, I was like, 'This looks good, this looks good.' On the last page, she wrote, 'You're going to be a dad!' and I skipped right over it. Then she said, 'Wait, hey, you missed it!' Then she pulled it back and I read it," Zach explained.
"We're going to be parents! I can't wait," Tori gushed to the cameras.