Police say the woman was attacked on Koh Tao, which draws divers from around the world and made headlines in 2014 when British backpackers Hannah Witheridge, 23, and David Miller, 24, were found bludgeoned to death on a beach.
French national Yohan Michael Tunka Buaga, 26, was arrested after the 18-year-old Briton told officers she was drugged and raped on 4 April.
Buaga was charged with rape on Monday and is being held on the nearby island of Koh Samui.
Police lieutenant colonel Piyapong Boonkaew said: “The suspect has admitted to having sexual intercourse with the victim, but he said that it was consensual and not rape.
“We are now investigating the case and waiting for forensic examination.”
Crimes against tourists in recent years have sparked concern about safety in the country, which remains a popular destination for those on a budget and relies on tourism to boost its economy.
Concerns over tourists travelling to Koh Tao were raised as early as August 2012 after British IT consultant Ben Harrington, 32, was found dead with both his watch and wallet missing.
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In 2015, Zaw Lin and Win Zaw Tun, migrant workers from Myanmar, were found guilty of killing Hannah Witheridge and David Miller.
Police said Ms Witheridge had been raped and bludgeoned to death and Mr Miller had died from blows to the head.