The Avengers star is in talks to join Steve Carell and Channing Tatum in the true crime drama, reports Variety.
The film will be based on the 1996 murder of Olympics wrestler David Schultz (Ruffalo) at the hands of his friend, chemicals business heir John du Pont (Carell).
Du Pont was a paranoid schizophrenic who ran a training centre called Team Foxcatcher in Pennsylvania.
Tatum will play Schultz's younger brother Mark, who was also an Olympic champion wrestler.
Foxcatcher will be the follow-up to Miller's Oscar-nominated 2011 film Moneyball.
Capote writers E Max Frye and Dan Futterman are working on the script.
Ruffalo is due to appear opposite Avengers co-star Scarlett Johansson in Can a Song Save Your Life?
Foxcatcher is due to begin shooting in October.
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