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 last year.
Both defendants were "guilty of murder for which the penalty is the death sentence," an unnamed judge reading the bench's verdict told the court on neighbouring Koh Samui.
The judge said "DNA evidence from both suspects" guided the decision, citing forensic traces from the accused found on Witheridge's body.
The migrant workers, who were shackled in court, were grim-faced as the verdict was delivered, while the mother of one of them wailed from the gallery.
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Both men denied killing the British pair and their lawyers have pledged to appeal after a police investigation they say was riddled with flaws and inconsistencies from the outset.
Throughout the trial prosecutors said their evidence against the men was rock solid.
But the defence has accused the police of bungling and using the migrant men - both aged 22 - as scapegoats.
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.
Miller was struck by a single blow and left to drown in shallow surf while Witheridge was raped and then bludgeoned to death with a garden hoe.
"He (David) was hacked down from behind, dragged into the sea and left to die. That will live with us forever. What happened to Hannah Witheridge is unspeakable," he said.
"David always stood up for justice and justice is what has been delivered today," he added, endorsing the work of the Thai police and the reliability of the forensic evidence.
Throughout the trial the defence disputed the forensic evidence and accused the police of torturing their clients into signing confessions, which they later retracted.