US Ambassador Glyn T Davies on Wednesday expressed concern about the "unprecedented" prison terms handed down under the Criminal Code Section 112 law.
No one should be jailed for peacefully expressing their opinion, he said during a talk at the Foreign Correspondents' Club of Thailand.
"We're also concerned by the lengthy and unprecedented prison sentences handed down by Thai military courts against civilians for violating the lese majeste law," Mr Davies told a sold-out event.
He also expressed worries about the way criminal defamation laws are more widely being used to stifle public debate.
Before asserting his point on the right to express opinions freely, Mr Davies stressed the deep respect and admiration the US held for the monarch.
"We believe no one should be jailed for peacefully expressing their views and we strongly support the ability of individuals and independent organisations to research and to report on important issues without fear of retaliation," he said.
He is in just his ninth week as ambassador,
Under the lese majeste law, a conviction for insulting the King, Queen, heir or regent faces up to 15 years in prison. Longer sentences are given if there is more than one offence, and if prosecutors use additional laws, such as the Computer Crime Act.
Since last year's coup the military has stepped up its patrol of alleged lese majeste offences, especially on social media. Analysts have concluded harsher prison sentences are being given than in the past.
In August the United Nations said it was "appalled" by the record jail sentences of 30 and 28 years handed to two Thais for royal defamation for "insulting" the monarchy on Facebook.
All media in Thailand including the foreign press routinely self-censor reports concerning the monarchy.
The law allows anyone to launch a complaint, and police are duty-bound to investigate. Critics have said lese majeste and criminal defamation laws are often used to pursue political opponents.
Thailand is a longtime ally of the US but the relationship between the two nations has been strained since last year's coup, which Washington strongly condemned.
Maintaining these ties has posed a delicate balancing act for the US, which is reluctant to isolate its old friend in the region.
On Wednesday Mr Davies reiterated the US call for a return to democracy in Thailand, but stressed he did not want to "come across as wagging a finger".
"Thailand has to do it on its own," he said.
