The Gulf carrier, which is backed by the Qatari state, now owns 25.1% of IAG after growing its shareholding from 21.4%.
No sum for the deal was disclosed when it was announced on Wednesday (19 February).
Qatar Airways chief executive Akbar Al Baker said: “Our investment to date has been highly successful and the announced increase in our shareholding is evidence of our continued support of IAG and its strategy.
“Qatar Airways continues to consider opportunities to invest in airlines and support management teams that share our vision to enhance travel opportunities for airline passengers across the globe.”
It comes after IAG last month removed its cap on non-EU shareholdings.
Last February, ownership of IAG’s shares by non-EU persons or bodies reached 47.5% where it was capped by the group.
This, however, had fallen to 39.5% by mid-January 2020 whereby IAG lifted the restrictions on share ownership.
Elsewhere, IAG chief executive Willie Walsh last month announced he would stand down on 26 March and leave IAG altogether in June.
He will be replaced by Iberia chief executive Luis Gallego.
