The pair are expected to appeal the verdict.
The brutality of the murders on the 2000-hectare island popular with divers and partygoers severely damaged Thailand's tourist industry.
Witheridge, from eastern England and her friend David Miller from the Channel Islands were bludgeoned to death with a rusty hoe after attending a late-night beach party on September 14, 2014.
As dawn broke Witheridge was found with her face smashed in, her skirt wrenched up and showing signs of rape.
Miller was four metres away, face up, with a blow to the head and water in his lungs.
Zaw Lin and Win Zaw Htun, who were working on the island, were arrested after weeks of intense pressure on police from Thailand's military rulers, the media and diplomats to solve a crime which shocked the Thai public.
Police said the pair had confessed to the killings but both men later retracted their statements, saying they were tortured.
Allegations of police incompetence and evidence mishandling dominated a 21-day trial on the nearby tourist island of Samui which ended in October.
Rights groups said the case reflected a trend of low-paid migrant workers being blamed for crimes in Thailand, where the justice system is seen as being riddled with corruption.
One senior Thai police officer told journalists "no Thai could possibly commit such a crime".
Prosecutors told a panel of three trial judges that DNA traces found on Witheridge's body as well as the suspect's being in possession of Miller's mobile phone and sunglasses proved the guilt of the accused.
But defence lawyers disputed the forensic evidence, saying DNA on the murder weapon did not match either of the accused and that the evidence-gathering techniques did not meet accepted international standards.
Prominent Thai forensic expert Porntip Rojanasunan criticised the collection of evidence, testifying that it "contradicted the principles of forensic science".
Authorities said initially that Witheridge was raped, then said she wasn't, before finally saying she was raped twice.
The case was mired in controversy.
Thailand's military ruler Prayuth Chan-ocha caused uproar when he commented on the killings, saying only ugly women were safe wearing bikinis in Thailand.
Police had Zaw Lin and Win Zaw Htun re-enact their alleged crime on the beach, with a television reporter roped in to play the parts of the victims.
As police linked arms to keep onlookers away, the men wielded a dustbin to supposedly demonstrate how they killed the tourists.
"The prosecution case was marked by an absence of significant evidence needed to prove the guilt of the accused for the crimes they are charged with," defence lawyers said on the eve of the verdict.
The lawyers said the accused men had no lawyers present when they were interrogated, they were not read their rights and DNA samples were taken from them involuntarily.
The prosecution did not present photographs of the crime scene, autopsy or DNA analysis processes and there was a lack of evidence on forensic laboratory procedures, they said.
Lawyers questioned why there was no evidence of forensic tests on Witheridge's clothes, CCTV footage provided by the prosecution appeared to be incomplete and no fingerprint or footprint evidence was presented as part of the prosecution case.
A report by British detectives who travelled to Thailand to review the evidence has not been made public.