The 35-year-old singer and television personality and 31-year-old actor announced the news Thursday on Instagram.
Simpson and Ross already have a 4-year-old daughter, Jagger Snow. Simpson is also parent to an 11-year-old son, Bronx, with her ex-husband Pete Wentz.
Simpson shared a photo Thursday of herself and Ross smiling and holding up their positive pregnancy test. She encouraged fans to donate to the March of Dimes Mom and Baby COVID-19 Intervention and Support Fund.
"We are pregnant and we are so excited to share it with everyone. Baby #3," Simpson captioned the post.
Ross posted the same picture on his own account.
"The fam is growing. Ash and I can't wait to welcome the newest addition," he wrote.
Simpson and Ross shared a video with their daughter this week that showed the family applauding healthcare workers on the frontlines during the coronavirus (COVID-19) pandemic.
"Thank you thank you thank you to all the healthcare staff and everybody on the frontline!! You are our heroes! We salute you!" Simpson wrote.
Simpson, the sister of singer and fashion designer Jessica Simpson, and Ross, the son of singer Diana Ross, married in August 2014. The couple released their first duet together, titled "I Do," in 2018.