Lang has played 44 games with the Cats, since being selected with pick No. 16 in the 2013 National Draft.
He enjoyed his best season in his second year at the club, playing 20 matches in 2014. However, he has struggled to maintain his spot in the senior side since, playing 23 matches over the past two seasons.
The 21-year-old’s decision could throw a spanner in the works for Geelong’s bid to bring Gary Ablett back to the Cats, with Lang seen as a possible player who could be involved in the trade given the Suns’ interest in him.