SYDNEY, Australia — The British-Australian mining giant said on Thursday that its chief executive, Sam Walsh, would retire on July 1 and that Jean-Sébastien Jacques, chief executive of its copper and coal division, would take the reins of the company, which faces the worst downturn in the sector in nearly two decades.

Rio Tinto, the world’s second-biggest mining company, after BHP Billiton, bowed to pressure from investors and credit ratings agencies this year and scrapped its payout policy, under which it had promised never to cut its dividend from year to year, to better reflect commodity cycles.

“The board has decided that J.-S. is the right person to lead Rio Tinto in an increasingly complex world filled with both challenges and opportunities for our industry,” Jan du Plessis, Rio Tinto’s chairman, , referring to Mr. Jacques.

Mr. Walsh, 66, joined Rio Tinto in 1991 after two decades in the auto industry, and was appointed chief executive in 2013 after the London-based company dismissed Tom Albanese over acquisitions that soured and prompted large write-downs. Mr. Walsh had been expected to leave this year, but it was unclear whether he would be replaced by Christopher Lynch, the chief financial officer, or by a younger division leader.

Mr. Jacques, 44, has spent more than three years on Rio Tinto’s executive committee. He was tasked with bringing greater focus to the copper and coal businesses and has been credited with improving safety while significantly reducing costs as the global mining sector faltered.

Most recently, the French-born executive took the lead role in negotiations over a $4.4 billion financing agreement necessary for an expansion of the giant Oyu Tolgoi copper mine in Mongolia.

Mr. Jacques will join the board and becomes deputy chief executive effective immediately, the company said in a statement.