US Defense Secretary Leon Panetta said the new joint defence declaration between the decades-old allies had "moved this alliance into the 21st century".
The last time the two nations drafted such a joint declaration was in 1962, when the United States promised to defend Thailand from "communist aggression".
Although the new statement made no reference to China's rising military power, Washington's strategic tilt to the Asia-Pacific is meant to offset Beijing's clout and maintain American influence there.
The United States promised "an enduring presence in the Asia-Pacific" and recognised Thailand as a "regional leader".
The Pentagon chief's trip to Asia has been overshadowed throughout by a snowballing sex scandal in Washington that forced the resignation last week of ex-general and CIA director David Petraeus over an extramarital affair.
The US commander in Afghanistan, General John Allen, has been linked to a key figure in the case and is now under investigation for potentially inappropriate emails.
Panetta said Thursday he was "not aware" of any other officials linked to the scandal.
His visit to Bangkok marked the first face-to-face talks between US and Thai defence ministers since 2008, and came days before President Barack Obama is due in the region for a tour of Thailand, Cambodia and Myanmar.
Obama will be the first sitting US president to visit Myanmar, following a series of dramatic political changes in a country emerging from decades of military rule.
He will meet President Thein Sein and opposition leader Aung San Suu Kyi.
A senior US defence official told reporters travelling with Panetta that the United States was open eventually to restoring military ties with Myanmar, but that the Pentagon would proceed cautiously.
"We're going to go slow. We are going to engage those we think are reformist elements," said the defence official, who spoke on condition of anonymity.
US officials were considering cooperating with Myanmar's armed forces on non-lethal programmes focused on military medicine, education and disaster relief exercises.
The activities would be "limited in scope" at the outset, the official added. "We'll grow as appropriate over time. We need to see reform, we need to see continued progress."
Washington restored diplomatic relations with Myanmar and ended sanctions on investment in July.
The United States has also dropped its objection to inviting Myanmar to observe Cobra Gold, the largest US multilateral exercise in the Asia-Pacific. It brings together thousands of troops from the US, Thailand and other countries for field training.
Thailand's air bases and ports remain vital to the US military's logistical network in Asia and the Pentagon continues to hold dozens of drills every year with Bangkok.
The United States suspended military aid to Thailand after a 2006 coup but reinstated it after elections in December 2007.
Earlier during his week-long trip to Asia, the third since June, Panetta took part in annual strategic talks with Australia in Perth, where officials unveiled plans to station a powerful US Air Force radar and space telescope.
He will fly to Cambodia on Friday to join a meeting of defence ministers from the Association of Southeast Asian Nations that is expected to focus on territorial tensions with China and recent sectarian unrest in Myanmar.