Its Prime Minister Xanana Gusmao joined ASEAN leaders for a summit and signing ceremony in the Malaysian capital of Kuala Lumpur, before posing for photographs together in interlocking arms.

"This is not only a dream realised, but a powerful affirmation of our journey -- one marked by resilience, determination and hope," Xanana said.

"This is not the end of our journey. This is the beginning of an inspiring new chapter," he added.

An emotional Gusmao said it was a historic moment for his country, with a new beginning that would bring "immense opportunities" for trade and investment. 

"Our accession is a testament to the spirit of our people, a young democracy, born from our struggle," he said, adding, 

Malaysian Prime Minister Anwar Ibrahim, whose country currently chairs ASEAN, said Timor Leste's accession "completes the ASEAN family -- reaffirming our shared destiny and deep sense of regional kinship".

"Within this community, Timor-Leste's development and its strategic autonomy will find firm and lasting support," Anwar said.

Timor Leste is the youngest country in the region, having gained independence from Indonesia in 2002 after 24 years of occupation.

President Jose Ramos-Horta has long campaigned for ASEAN membership, and an application was first submitted in 2011 during his first term.

Ramos-Horta, 75, who was awarded the Nobel Peace Prize in 1996, had raised the idea of Timor Leste joining ASEAN back in the 1970s, to secure his country's future through regional integration. 

In an interview with CNA in September, Ramos-Horta said Timor Leste must maintain stability and not burden ASEAN, adding it could contribute with its own experiences of conflict, including for disputes over borders and the South China Sea.

"If we can in the future contribute towards strengthening ASEAN mechanisms such as conflict mechanisms, that is key, in each country in ASEAN, we put emphasis on dialogue," Ramos-Horta said.

The country's admission is seen as one of the crowning achievements of Malaysia's ASEAN chairmanship.

Timor Leste was granted observer status to the regional body in 2022 but its full membership was delayed by various challenges.

Timor Leste faces challenges in infrastructure development and human resource capacity, seen as critical for effective participation in ASEAN's economic community.

In September, thousands of student-led protesters demonstrated against a multi-million dollar plan to purchase Toyota Prado SUVs for each of the country's 65 members of parliament and lifetime pensions for former MPs.

Demonstrators and police clashed for two days, before the parliament cancelled the vehicles' procurement.

The parliament has also bowed to public pressure over MPs' pensions.

ASEAN began as a five-member bloc in 1967 and has gradually expanded, with Cambodia the most recent addition in 1999.