Speaking to PEOPLE, the couple said: "It is with tremendous sadness that we have come to the decision to divorce.
"We enter this next phase with continued friendship, love and respect for one another and the shared focus of caring for our son.
Read more: Patrick Stewart keeps bag of James McAvoy's hair in a safe at his home
The pair first met while working on Shameless, in which Duff played the love interest of her future husband's character.
Speaking a year after their wedding, McAvoy revealed they liked to have "a normal life".
He told The Washington Post: "We don't talk about each other much and we don't turn up together at lots of events. We both really rejoice in having a normal life."
