Zaw Lin and Wai Phyo killed Hannah Witheridge, 23, from Norfolk, and David Miller, 24, from Jersey, judges said.
The backpackers' bodies were found on a Koh Tao island beach in September 2014.
The defendants' lawyers say they will appeal. The accused retracted their initial confessions, saying police had tortured them. Mr Miller's brother said justice had now "been delivered".
Miss Witheridge's family said they needed time "to digest the outcome of the trial verdict".