The pair have both been given the death sentence after the judges ruled that Zaw Lin and Wai Phyo were responsible for the killings.
The bodies of the backpackers were found on a beach on the island in 2014.
The defendants’ lawyers have said that they will appeal the decision, the BBC reported.
Lin and Phyo were bar workers on the island having travelled over from Burma.
Prosecutors said that DNA evidence linked them to the deaths.
But lawyers defending them said that a DNA sample on a garden hoe, which was allegedly used as a murder weapon, did not match samples given by the pair.
Andy Hall, international affairs adviser for Migrant Worker Rights Network, which represented the men, said: "We strongly disagree with the decision of the court. This investigation was a shambles from the beginning.
"The defence team have had access to all the information in this case and the information we saw did not comply with international standards."
Speaking outside court, Mr Miller’s brother Michael said: "We believe the result today represents justice for David and Hannah.
"It is our opinion that the evidence against Wai Phyo and Zaw Lin is absolutely overwhelming. They raped to satisfy their selfish desires and murdered to cover up that fact. They have shown no remorse during the trial."
Witheridge’s family did not travel to the island of Koh Samui, where the trial took place, to hear the verdict.
In a statement they said: "The past year has served as an unimaginably impossible time for our family. We have found the trial process extremely difficult and our trips out to Thailand, to attend court, made for particularly distressing experiences.
"We found listening to proceedings very challenging and we have had to endure a lot of painful and confusing information. We now need time, as a family, to digest the outcome of the trial and figure out the most appropriate way to tell our story."
