Patong Police were alerted by Patong hospital at 5.40pm of the death of Grigorii Baksheev, 54, a Russian national.
At hospital, officers met a Russian woman, Natalia Sharapova, 44, who was named as a friend who had been staying with the deceased at a hotel at Tri Trang Beach, near Patong.
The two had checked in on March 19, and were due to check out March 27, the woman told police.
However, at about 4.30 on Saturday, Mr Baksheev reported chest pains while taking a shower, and when he collapsed, Ms Sharapova called for help from hotel staff, who attempted CPR on the man before an ambulance was dispatched.
Doctors' efforts to revive the man were unsuccessful and an initial examination confirmed "severe heart failure" as the primary cause of death.
A subsequent inspection of the hotel room found nothing unusual, though inspectors collected a feces sample from the bathroom floor.