The miner said it had compulsorily acquired the remaining 7.9m shares not voluntarily offered after it made a $C654m (£419m) takeover bid in October.
Hathor, which is based near western Canada’s Athabasca Basin, supplies approximately a fifth of the world’s uranium.
Rio Tinto employs more than 13,000 people in Canada in aluminium, iron ore, diamond and titanium dioxide operations.
The takeover marks the end to a bidding battle for Hathor in which Cameco also made an offer for the company. Last week Rio announced that it had acquired over 93.7 per cent of Hathor and that it intended to acquire the remainder by a compulsory acquisition under the Corporations Act.