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."

Bribes of between $1,000 - $5,000 (£800; £4,000)  have been paid, Gen Nur Farah Jimale said.
The BBC has also learned that some candidates have been offering bribes of up to $1.3m (£1m) to secure votes.
Indirect elections have been taking place since October as the country remains too unsafe for a national vote.
Much of the country is still under the control of Islamist militant group al-Shabab, which is affiliated to al-Qaeda.
Gen Nur told the BBC Somali Service that some seats should be re-run because money had been used to "out-manoeuvre" other candidates, in some cases, meaning all but one candidate had withdrawn.
Cases of government resources being used in the election have also been documented, he said.
The elections are being funded by the United Nations which also supports the government based in the capital, Mogadishu.
Somalia has not had a functional national government since the ousting of its former leader Mohamed Siad Barre in 1991.
Veteran Celtic rockers Runrig are to appear in a new Scottish film and will also compose the movie's title music.
[NEXT_CONCEPT]
Massive cases of corruption and voter intimidation have marred the ongoing parliamentary elections in Somalia, the auditor general has told the BBC.