At Sunday's meeting with Samak Sundaravej, Thailand's prime minister, and a dozen senior military officers, Gates said America's relationship with Thailand was "based upon shared democratic values".
An unnamed official travelling with the US defence secretary told reporters the message was "not hectoring" but was "clear, and respectful".
In February the US lifted restrictions on aid to Thailand after Samak's democratically elected government took power.
Washington suspended about $35m in annual assistance, including funds designed to promote military professionalism, after the 2006 coup that topped Thaksin.
Thailand has seen at least 18 coups or attempted coups since the end of rule by absolute monarchy in 1932.
|Gates met the Thai prime minister and leading |
military officers on Sunday [AFP]
The weekend meanwhile saw further street protests against Samak's government, in defiance of government threats to use for to break up demonstrations.
On Saturday Samak had threatened to use the police to break up a rally of 6,500 anti-government protesters.
But police did not move against them after an apparent reversal by the prime minister, who accused the media of misreporting his remarks.
Protests continued on Sunday but with numbers down to the hundreds rather than thousands.
Analysts say violence stemming from a clampdown on protests could add to concerns about the army being drawn back into the political fray.
Last week Thailand's military commander denied reports that he had implied the unrest could trigger a coup attempt.
Supreme Commander Boonsrang Niumpradit had been quoted as saying that he could not guarantee members of the armed forces would not launch a coup, but he later said his remarks had been taken out of context.