Turquoise Hill said on Thursday Ulf Quelmann resigned, effective March 3, after Rio said it would vote against his re-election to the board.
Pentwater, Turquoise Hill's largest shareholder after Rio with a 9% stake, said it believed the boards of Turquoise and Rio breached their legal obligations in the way Quelmann's departure was handled.
In response to Pentwater's statement, a Rio spokesman said the company supports the reset of leadership at Turquoise.
Turquoise Hill did not immediately respond to requests for comment.
During Quelmann's over two-year tenure as CEO, Turquoise found itself locked in a feud with Rio over how to fund the underground expansion of the massive Oyu Tolgoi copper-gold mine in Mongolia.
Rio is the operator for the mine and owns 51% of Turquoise Hill, which in turn owns 66% of Oyu Tolgoi.
Pentwater did not detail what actions it plans to take and did not immediately respond to a request for comment.
(Reporting by Shariq Khan in Bengaluru; Editing by Sriraj Kalluvila)