A one-sentence statement released Monday says the actors are "thrilled" to have another baby on the way. The couple has two daughters, 5-year-old Violet and 2-year-old Seraphina Rose Elizabeth Affleck.
No other details were released.
Affleck and Garner, both 39, were married in 2005.
Garner, who rose to fame after starring in the television series "Alias," appeared over the weekend at a Disney expo to promote her upcoming film, "The Odd Life of Timothy Green."
Affleck won an Oscar for co-writing the screenplay to "Good Will Hunting" and recently wrote, directed and starred in the thriller "The Town."