The state-owned Gulf carrier said it had acquired a 9.6 per cent stake in Cathay, which would make it the third largest shareholder in the Hong-Kong based airline.
Bloomberg News said the deal was worth USD 662 million.
Qatar Airways group chief executive Akbar Al-Baker said in a company statement that Cathay was "one of the strongest airlines in the world", adding it had "massive potential".
The acquisition comes during the ongoing political crisis in the Gulf, the worst to hit the region in years.
As part of that dispute, Qatar Airways has been stopped from flying to Saudi Arabia and the UAE, previously popular destinations for the carrier.
The deal with Cathay Pacific also comes after Qatar Airways halted a bid in August to buy a 10 per cent stake in American Airlines.