"Hilary and Chad have decided to separate, but they are hopeful they'll be able to get through this tough time," Swank's manager Troy Nankin said in a statement Monday.
There was no elaboration on the reason for the split.
Swank, 31, and Lowe, who turns 38 on Jan. 15, were married on Sept. 28, 1997. They have no children.
Swank famously forgot to thank a tearful Lowe while accepting her best actress Oscar in 2000 for "Boys Don't Cry." Last year, Swank won again for "Million Dollar Baby" — this time, thanking her husband.
She will next star in "The Black Dahlia," Brian De Palma's adaptation of James Ellroy's novel.
Lowe, who is the brother of actor Rob Lowe, won an Emmy Award for his portrayal of AIDS patient Jesse McKenna on the TV series "Life Goes On" in 1993.
David Rose, the actor's agent at Innovative Artists, said there would be no comment from Lowe.