The suit relates to a contract Amaranth and Touradji signed in September 2006 covering the purchase and transfer of Amaranth's base metals portfolio.
Touradji, a big commodities hedge fund run by Paul Touradji, generated profit through "improper" trading and misuse of proprietary and confidential information from Amaranth about its base-metals portfolio, the lawsuit claims.
Amaranth is trying to recoup $350m from Touradji, according to the suit, which was filed Monday in New York State Supreme Court.
Amaranth lost more than $6bn from huge natural gas trades that went bad in 2006. The fund was shut down soon after.