The 37-year-old actress and 36-year-old singer announced the news in a statement to People magazine Thursday. The couple already has two daughters: 7-year-old Naleigh and 4-year-old Adelaide.
"The Kelley clan is thrilled to announce that we are expecting a third addition to our family," the pair said. "Naleigh and Adelaide could not be more excited to welcome their new sibling into the fold and Josh and I are overflowing with joy and gratitude."
"This is an incredibly exciting time for us filled with hope, anticipation, and hormones," Heigl added. "We couldn't be more excited to finally share this news with our fans and are looking forward to continuing to share the journey with you all."
Heigl and Kelley married in 2007 after more than two years of dating. They adopted Naleigh from South Korea and Adelaide in the United States. Their son is due in January 2017.
"Hahaha!! Well I guess the cats out of the bag!!!! We're so excited people!!! What an awesome journey!!! Can't wait to hold this little nugget," Kelley wrote on Instagram after the pregnancy announcement.
Heigl is best known for playing Izzie Stevens on Grey's Anatomy and will star with Steven Pasquale in the new CBS series Doubt. Kelley last released the album New Lane Road in April.