The Trade and Investment Framework Agreement is aimed at promoting bilateral dialogue and cooperation on trade and investment issues, the USTR said in a statement.
"As part of this dialogue, the two sides will work together to identify initiatives that support the ongoing reform program and promote inclusive development that benefits the people of Burma, including the poorest segments of its population," the trade office said, using the former name of Myanmar.
The trade pact was signed by Acting US Trade Representative Demetrios Marantis and Myanmar's deputy commerce minister, Pwint San.
Thein Sein, who took office as a nominal civilian in 2011, has surprised even cynics by freeing hundreds of political prisoners, reaching ceasefires with ethnic rebel groups, easing censorship and letting long-detained opposition icon Aung San Suu Kyi enter parliament.
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The Obama administration suspended most sanctions on Myanmar last year and is hoping the trade agreement will show the country tangible benefits for embracing reform.
"The United States supports reforms that lay the foundation for a peaceful and prosperous future," Marantis said.
"Stronger institutions, transparency, and rule of law create stronger foundations for commercial transactions, trade and investment."
The new trade agreement addresses the continuing US and international concerns about labor rights in the impoverished country, the USTR said.
Under the pact, the United States said it would seek to work with the Myanmar authorities to achieve better protection of worker rights.
According to US official data, bilateral trade totalled USD 90 million in the first three months of 2013, including USD 89 million in US exports to Myanmar.