The series follows Ted Mosby (Josh Radnor), who in the year 2030 recounts to his children the events that led him to meeting their mum. Former O.C. star Bilson previously featured in an episode of the show's fifth season as Cindy, the future mother's roommate.
Entertainment Weekly reports that she will reprise her role in the upcoming sixth season premiere, when Cindy arrives at MacLaren's bar, a favourite haunt of the show's characters.
Series creator Carter Bays said: "The season premiere is going to be very much informed by [Cindy's] presence."
The new season of How I Met Your Mother will begin on CBS on September 20.