The murder occurred on Friday, reported the Aftonbladet newspaper, citing a local newspaper in tourist resort Pattaya, south of Bangkok.
According to English language newspaper Pattaya Daily News, the Swedish boy was in the country visiting his girlfriend in the city Amphur Muang in eastern Thailand.
The girlfriend, in turn, had made contact with the other 18 year-old, a local student, through Facebook, and brought him to the Swede’s hotel for the two to meet.
Once at the hotel, however, the student rushed in to the Swede’s room, and left the girl locked outside. Sounds of fighting were heard through the locked door, and some minutes later the student opened the door covered in blood, and left.
Police found him lying dead outside the hotel.
Thai authorities have informed Sweden’s Foreign Ministry (Utrikesdepartementet, UD) of the arrest, and the ministry’s press manager Anders Jörle told news agency TT that the arrested Swede is not yet 18 years old, and therefore still a minor.
“We’ve been informed that he’s been arrested for this investigation, and we know what it’s about. There isn’t much more to say about it,” Jörle said.
According to Pattaya Daily News, local police found the Swedish boy sitting with his girlfriend in his hotel room. He has confessed to dealing the deadly stabs, but claims they were in self-defense.
His room showed clear signs of a fight, and there were several pools of blood on the floor. Police also found a roughly fifteen centimetre long knife.
The Swede reportedly met his 18 year-old girlfriend in Sweden where she lives with her Thai mother. She had been spending some time in her former home town Amphur Muang, and the Swedish boy checked in to hotel Jantra in Amphur Muang four days ago.