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," a Thai court ruled.
Police arrested the two workers soon after the incident and said the two had confessed to the crimes.
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.
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The judge said "DNA evidence from both suspects" guided the decision, citing forensic traces from the accused found on Witheridge's body.
After a lengthy trial the judge today declared them guilty and sentenced them to death. The two men are expected to appeal.
Witheridge's family said they needed time "to digest the outcome of the trial verdict".
"We strongly disagree with the decision of the court. This investigation was a shambles from the beginning," Andy Hall, international affairs adviser for Migrant Worker Rights Network, which represented the men was quoted as saying by BBC.
He said the defence team would mount an appeal in the case.