Until recently, South Africa was the only African country that was part of the grouping.
"Kenya welcomes the addition of the African Union – the fastest growing continent in the world – to the G20," Kenyan President William Ruto wrote on X, formerly known as Twitter. "This will increase the voice of Africa, visibility, and influence on the global stage and provide a platform to advance the common interest of our people."
"With Africa poised to grow in the coming years, a seat will allow it to shape the decisions of G20 to ensure the continent's interests are advanced," Ruto said in a sperate statement.
"The outcome of the just concluded Africa Climate Summit including fundamental reforms of international financial institutions and multilateral development banks is one thing that AU will advance," he added.
AU Commission head Moussa Faki Mahamat said earlier that the move will allow the continent to make "its effective contribution" in helping the world meet global challenges.
"This membership, for which we have long been advocating, will provide a propitious framework for amplifying advocacy in favour of the Continent and its effective contribution to meeting global challenges," he wrote on X.
Comoros president and current AU chairman Azali Assoumani said "it's a great day for me... but also for the whole of Africa, which has just joined the G20, with great emotion and pride".
The African Union at full strength has 55 members but six military-ruled nations are currently suspended. Collectively, it has a GDP of $3 trillion with some 1.4 billion people.
As the G20, the grouping included 19 countries and the European Union, representing 85 percent of the world's GDP, with South Africa its only member state from the continent.
South Africa's President Cyril Ramaphosa said on X, formerly known as Twitter, that he was "delighted" by the move.
Issues around climate change "can only be addressed collectively and with a great deal of solidarity" he added, and "access to adequate and predictable financial resources from a variety of public and private sources is critical if we are to achieve the Sustainable Development Goals".
"As a continent, we look forward to further advancing our aspirations on the global stage using the G20 platform," Nigerian President Bola Ahmed Tinubu, who is at the summit, posted on X.
Senegal's President Macky Sall also welcomed the African Union's inclusion in the G20, for which he had long advocated.
"I warmly thank all the members of the G20 for their support for this initiative, which I championed during my term at the head of the African Union", said Sall, who assumed the AU presidency in 2022 until the beginning of 2023.
This advocacy "reflects his vision of fairer and more inclusive global political, economic and financial governance," Senegal's foreign ministry said in a statement.