Among the provisions of the new charter is the creation of a regional agency to monitor human rights.
But it has been criticised by rights groups as lacking teeth because it contains no provisions to suspend or eject a member state.
Critics say, for example, that the new agency will have no powers to punish notorious violators of human rights such as the military government of Myanmar.
Myanmar has said it will join the nine other Asean members signing the new charter on Tuesday after references to punishments for violators were taken out of the final draft of the document of the constitution.
Myanmar's military rulers launched a bloody crackdown on anti-government protests in September, leaving at least 15 people dead and an unknown number in jail.
The crackdown was seen as an embarrassment for Asean, and the group has faced growing international pressure to become a force for change in the country.
"The reputation and credibility of Asean has been called into question"
US trade representative
Susan Schwab, the US trade representative, told reporters on Monday that Asean had "a special responsibility" when it came to finding a solution to Myanmar's political crisis.
"The reputation and credibility of Asean has been called into question," she said after a meeting with Asean economic ministers in Singapore.
Her comments came a day after the head of Asean, Ong Keng Yong, rejected a US senate resolution calling for the group to suspend Myanmar's membership.
"Our approach is not to take such a confrontational, drastic action, especially when it doesn't yield good results," he said.
Traditionally Asean has stood by a principle of non-interference in member state's affairs – a stance which has led some critics to label it a powerless talking shop.
With the new charter though, Asean officials say the group will become a rules-based legal entity, more along the lines of other regional groupings such as the European Union.
Commenting on the constitution which took more than two years to draft Syed Hamid Albar, Malaysia's foreign minister, said the new document would "give substance to Asean after 40 years of our existence".
He added that a meeting of Asean foreign minister on Monday, a day ahead of the summit opening, had agreed it should be ratified within one year.