BHP Billiton said the sale of its share in the South African mineral sands miner and smelting operator "reflects the company's commitment to a simpler, more scalable upstream portfolio."
Rio Tinto had already owned a 37 percent stake in RBM and managed its operations and marketed its products.
BHP had triggered the sale in February when it exercised an option to sell its stake agreed during a 2009 restructuring of RBM.
"Doubling our stake in this tier one asset further strengthens Rio Tinto's titanium dioxide portfolio at a time when the long-term outlook remains robust," said Rio Tinto Diamonds & Minerals chief executive Alan Davies.
In 2011, RBM had a 14 percent share in global sales of titanium dioxide feedstock, accoring to Rio Tinto. The mineral is used in products from sunscreens to food colourings.