Indonesia's candidacy was endorsed by BRICS leaders in August 2023, according to the foreign ministry of Brazil, which holds the group's presidency for 2025.
However, the world's fourth-most populous nation opted to formally join the bloc only after the formation of its newly elected government in 2024.
"The Brazilian government welcomes Indonesia's entry into the BRICS," the government said in a statement on Monday.
"With the largest population and economy in Southeast Asia, Indonesia shares with other members a commitment to reforming global governance institutions and contributes positively to deepening South-South co-operation."
BRICS was formed by Brazil, Russia, India and China in 2009, and South Africa was added in 2010.
In 2024, the alliance expanded to embrace Iran, Egypt, Ethiopia and the United Arab Emirates. Saudi Arabia has been invited to join but has not yet done so.
Turkey, Azerbaijan and Malaysia have formally applied to become members and a few others have expressed interest.
The organisation was created as a counterweight to the Group of Seven, comprised of developed nations.
Its name derives from an economic term used in the early 2000s to describe rising countries expected to dominate the global economy by 2050.
Before Indonesia's membership, the bloc accounted for nearly 45 per cent of the world's population and 35 per cent of global gross domestic product, measured using purchasing power parity.
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