The miner finally paid C$43.00 a share for the remaining 49% stake in Turquoise Hill it does not already own after a long-drawn bidding war.
With the transaction now concluded, Rio Tinto holds a greater control in the Oyu Tolgoi project with a 66% direct stake.
The remaining 34% interest is held by the Mongolian government via Erdenes Oyu Tolgoi.
The deal completion follows clearance from the Yukon court in Canada and support from the majority of Turquoise Hill minority shareholders.
Claimed to be one of the largest known copper and gold deposits in the world, the Oyu Tolgoi underground development is expected to have production of 500,000 tonnes per annum of copper.
Rio Tinto CEO Jakob Stausholm said: “Oyu Tolgoi is an outstanding asset with incredible people that will deliver significant long-term value for Rio Tinto and Mongolia.
“I would like to acknowledge the Turquoise Hill Board and management, under the leadership of Peter Gillin and Steve Thibeault, for their considerable effort and support over the years, working in partnership with Rio Tinto and The Government of Mongolia in developing Oyu Tolgoi into a world-class mine.”
Turquoise Hill has now become Rio Tinto’s wholly owned subsidiary.
Rio Tinto copper CEO Bold Baatar said: “This acquisition further strengthens our copper portfolio, as part of our strategy to grow in materials the world needs for achieving net zero and delivering long-term value for our shareholders.
“We now have a simpler and more efficient ownership and governance structure, with our partner the Government of Mongolia, as we proceed together towards sustainable production from the underground mine.”