The company said it will make an offer to buy all of the common shares of Hathor for C$4.15 ($4.06) in cash per share.
The acquisition of Hathor allows Rio Tinto to expand its presence in the Athabasca Basin of northern Saskatchewan, Canada, which currently provides about 20% of global uranium production.
Rio Tinto Energy chief executive Doug Ritchie said the medium and long-term outlook for the uranium market is positive, with uranium assuming a significant role in the world’s primary energy needs.
"This acquisition will allow us to build on the platform successfully laid out by Hathor and we will continue to draw on their expertise and commitment going forward," Ritchie said.