According to local police, a passerby first noticed the woman, identified as Karen Pai, sunbathing face-down on Wednesday morning. When he returned about three hours later, he became concerned after noticing she had not moved and appeared to have bruises on her body.
He immediately alerted lifeguards and police, the Bangkok Post reported.
Police found a key card for Kata Palm Resort among the woman's personal items.
Pai had checked into the resort alone on Monday and was scheduled to leave on Thursday, according to The Nation.
Security footage from the resort showed Pai leaving her room around 10:23 a.m. on Wednesday. Forensic examinations estimated she had been dead for four to six hours before being discovered.
Authorities reported no signs of physical assault and suspected that she had died of heat stroke.
Phuket’s temperatures on Wednesday were between 26-34 degrees Celsius, according to U.S. weather forecast service AccuWeather.
Thai authorities have informed the Canadian embassy about the situation.