Commenting on the acquisition, Cathay Pacific CEO, Rupert Hogg said: "Qatar Airways is one of the world's premier airlines".
Qatar Airways will purchase the stake for HK$5.16 billion (€570 million) from Kingboard Chemical Holdings and associates, according to a statement, becoming Cathay's third-largest investor after local conglomerate Swire Pacific, with a 45 per cent holding, and Air China Ltd. with nearly 30 per cent.
The state-owned Gulf carrier said it had acquired a 9.6 percent stake in Cathay, which would make it the third-largest shareholder in the Hong Kong airline.
Qatar Airways' investment strategy since 2015 has focused on increasing stakes in airlines based outside the Middle East.
Despite Cathay's woes, Qatar Airways chief executive Akbar al-Baker described it as "one of the strongest airlines in the world. with massive potential for the future". This already includes a 20% stake in International Airlines Group, 10% in LATAM Airlines Group and 49% in Meridiana.
Cathay flew between Hong Kong and Qatar Airways' Doha hub as part of a codeshare arrangement between 2014 and 2016.
Cathay did not put out a statement, but tweeted that the Swire Group and Air China continue to hold 74.9% of the airline. On Monday, Air China said that it looked at the purchase by Qatar Airways in Cathay as positive and it hoped shareholders would work to find synergies.
In particular, the rapid growth of Chinese carriers on worldwide routes has reduced the number of passengers from China transiting through Cathay's Hong Kong hub.