SYDNEY, Australia — The mining giant said Wednesday that it had agreed to sell its 40 percent stake in an Australian coal mine for $606 million as it seeks to reduce debt by selling assets as commodity prices are mired in a global slump.

The New Hope Corporation, an Australian coal producer, bought Rio Tinto’s minority stake in Bengalia, the smallest of Rio Tinto’s three mines in the coal-rich Hunter Valley region of New South Wales.

“This sale will deliver value for our shareholders as we remain focused on continuing to develop the strongest core portfolio of assets in the mining industry,” Jean-Sebastien Jacques, chief executive of Rio Tinto’s Copper and Coal group, said in a statement.

Rio Tinto has sold $4.5 billion worth of assets since January 2013, when Sam Walsh became chief executive. Mr. Walsh has sought to sell the company’s assets to repair its balance sheet since he succeeded Tom Albanese as chief executive.

At the time of Mr. Albanese’s resignation, Rio Tinto took a $14 billion write-down on the aluminum and coal assets that it bought at the height of the commodity boom from 2000 to 2009.

Rio Tinto’s net debt as of June 30 was $13.68 billion, an increase of 10 percent from Dec. 31 last year. The company’s net cash generated from operating activities dropped 19 percent over the same period to $4.43 billion.

Mr. Walsh said last month that the company was operating in a “tough” environment. Rio Tinto’s earnings are dependent on the price it receives for its iron ore, which accounted for 72 percent of the company’s underlying earnings in the first six months of 2015.

The iron ore price has slumped more than 20 percent this year. Demand for Australian iron ore by Chinese steel producers has slumped. Miners have increased production, causing oversupply, just as China’s economy has slowed.

There is also oversupply in the coal market. The price for Australia’s Newcastle coal, an Asian benchmark, has fallen to less than $60 a metric ton from about $136 in 2011.