Put together in 1973, the band enjoyed UK chart success with Gaelic songs in the 1990s and last year released their 14th and final studio album.
Their song An Ubhal as Airde reached number 18 in the charts in 1995.
The film, Journey Bound, is a romcom road movie being directed by Ryan Hendrick and starring Clare Grogan, Sylvester McCoy and Karen Dunbar.
Runrig founding member Calum Macdonald said: "It's a refreshing and exciting challenge, so far removed from the familiarity of normal Runrig business."
Still Game's Ford Kiernan, who also stars in Journey Bound and is an executive producer, said: "I first saw Runrig perform live at Glasgow University Union in the 1980s whilst I was working there, the place was jumping, the atmosphere was incredible.
"It really is the icing on the cake for us and I'm over the moon they are onboard. They are the perfect musical fit for this quintessential Scottish movie."
Veteran Celtic rockers Runrig are to appear in a new Scottish film and will also compose the movie's title music.