By Agence France-Presse
Zaw Lin and Win Zaw Tun were found guilty of killing David Miller, 24, and the rape and murder of Hannah Witheridge, 23, whose battered bodies were found on a beach on the southern diving resort of Koh Tao in September 2015.
Their original conviction in December 2015 was mired in controversy.
Prosecutors insisted their evidence against the men was rock solid.
But the defence accused the police of bungling their investigation from the outset after local detectives came under huge pressure to solve a case that risked damaging the country's vital tourism sector.
Investigators were accused of failing to properly collect and preserve DNA samples and declining to test key pieces of evidence, such as Witheridge's clothes, or allow independent examination of the samples.
But a higher court threw out the defence's appeal.
"The Appeals Court upheld the Criminal Court's ruling against the two Myanmar suspects," Theerawut Phamhun, deputy provincial prosecutor of Surat Thani province, told AFP.
He said the appeal judges ruled the evidence investigation "were up to standard" and that DNA found at the scene matched the suspects.
The defence now have 30 days to appeal to Thailand's Supreme Court, the final court of appeal.
Zaw Lin and Win Zaw Tun's defence lawyer did not pick up his phone Wednesday.
While Thailand keeps the death penalty on its books it is rarely carried out.