The 66-year-old stood to read the Parliamentary oath in unison with 33 other members of her National League for Democracy party who were elected to the lower house in April, an AFP reporter said.
The signing of the oath marks a dramatic transformation in the fortunes of the 66-year-old who was held under house arrest for much of the past 20 years but is now central to the nation's tentative transition to democracy.
The oath, taken in front of lower house speaker Shwe Mann, states members will "safeguard and abide by the Constitution of the Union" and "hold always in esteem (the) non-disintegration of the Union, non-disintegration of national solidarity and perpetuation of sovereignty."
Ms Suu Kyi and other newly elected members of her National League for Democracy (NLD) party had initially refused to swear the oath which "safeguards" the army-created constitution.
But after its plea to the President Thein Sein's reformist government to amend the wording was unsuccessful, Ms Suu Kyi on Monday agreed to make the pledge and take her seat in the fledgling parliament in the capital Naypyidaw.
"The reason we accept (the oath), firstly is the desire of the people. Our voters voted for us because they want to see us in parliament," she said on Monday.
Members of the NLD -- the main opposition force after securing 43 of the 44 seats it contested in April 1 by-elections -- joined her at the Parliament.
Speaking in Yangon on Tuesday UN Chief Ban Ki-moon welcomed Suu Kyi's climbdown as a sign of the "flexibility" required to help resource-rich but poor Myanmar make the transition from half a century of a military-rule.
"I know that it must have been a very difficult decision," Mr Ban said, expressing confidence that her party and the government led by former general Thein Sein can co-operate.
The United States said it was also optimistic further steps towards democracy will be taken after the NLD agreed to take their seats in parliament.
"I think we see this as a hopeful sign that the (NLD) and the government will work together to continue to build on... the democratic progress that we've seen there," US State Department deputy spokesman Mark Toner said.
The NLD, which boycotted a controversial 2010 election, agreed to rejoin the political mainstream last year after a series of reforms by the government.
Ms Suu Kyi has vowed to push for an amendment of the 2008 constitution, under which one quarter of the seats in parliament are reserved for unelected military officials.
After spending much of the last two decades under house arrest, Ms Suu Kyi was freed in 2010 and her election to the Parliament has been greeted by the international community as a step towards democracy.
She has shown increased confidence in Thein Sein's government in recent weeks, calling for the suspension of EU sanctions and planning her first international trip in 24 years.
Last week, European Union nations suspended most sanctions against the impoverished nation for one year to reward the reforms, which included releasing some political prisoners.