Karolina Boronczyk landed on an overhang on the first floor, said Pol Lt Siwa Klabphan, an investigator at Phayathai police station.
Susanna Boronczyk, the deceased's sister, told authorities that she and Karolina had been staying in room 1051, which was adjacent to her parents’ room, at the Bangkok Palace Hotel since Tuesday and were due to check-out on Thursday morning.
Ms Susanna said she was sleeping when the incident took place.
There was no sign of a struggle or anything out of the ordinary inside the hotel room, said Pol Lt Siwa. He believed Karolina lost her balance while opening a window in the room and fell out.
Police are still investigating.