It will be the first marriage for both actors, Gyllenhaal's representative, Amanda Silverman, said Tuesday. No other information was released.
The couple have been dating for four years.
Screen credits for the 28-year-old Gyllenhaal include roles in Oliver Stone's upcoming "World Trade Center," "Mona Lisa Smile," "Secretary" and "Donnie Darko," co-starring her brother, Jake Gyllenhaal.
Sarsgaard, 35, starred alongside Jake Gyllenhaal in last year's war drama "Jarhead." His screen credits also include "Boys Don't Cry," "Shattered Glass" and "Kinsey."