Police in the coastal city of Pattaya said they found Tunchanok Donhomla "stuffed" in the bag, which had been discarded near a railway track, in the early hours of Saturday.
Thai police said they arrested Simon Peter Carman at Bangkok's Suvarnabhumi Airport in connection with the death as he was allegedly "preparing to flee the country."
He denies the charges. In a message issued to the victim's family after his arrest, Carman said: "I feel bad for what happened to your daughter. It was out of my control."
Pattaya City Police said the 17-year-old had been reported missing at 17:00 local time (12:00 GMT) on Friday.
In a statement on social media, the force said it reviewed CCTV footage that allegedly showed Carman entering a condominium with Donholma at 03:34 on Thursday. Late that evening, he emerged alone "carrying a large suitcase."
The statement said he loaded the bag onto a motorbike before driving towards a railway line.
Officers questioned and arrested Carman at the airport in Bangkok, some 150km (93 miles) north of Pattaya, at 01:15 on Saturday.
The teenager's naked body was found in a suitcase about 15 minutes later, the force said.
Carman denied murder and further charges related to moving or concealing a body and taking a minor for sexual purposes and claimed he had acted in self-defense.
Media reports state that Carman told police he had agreed to pay Donholma 1,000 baht (£23, US$30, A$43) for sexual services but that they had an argument when they returned to his apartment and he offered only 500 baht.
Thai police told the Australian Broadcasting Corporation (ABC) that Carman also claimed Donhomla had "disappeared from the room" while he was asleep.
"I know you'll be very sad, upset… same [as] me."
He added: "Please tell other girls… just to be careful."
Colonel Anek Srathongyoo, Superintendent of the Pattaya City Police Station, told the ABC that Carman "has fingernail scratches across his body that are consistent with a struggle, but he denies killing her".
In another police video clip Carman is seen being asked about the scratch marks.
"I think it's a spider; they always get in here," he replies.
Donhomla's family told the ABC she was an only child who lived with her father and stepmother in the province of Kalasin, about 480 km northeast of Pattaya. The teen, whose nickname was Cake, had told her parents she wanted to go on holiday with a friend and had travelled to Pattaya on 16 June.
Her father, Thongchai Donhomla, said he was "deeply saddened" by his daughter's death.
"My daughter had no mother, so whenever she wanted anything, she would find a way herself, and she always helped me too," he told a local reporter.
Her stepmother, Oradee Bussarakum, said, "We were scared. We just hoped it wouldn't turn out the way we feared. Now our eyes are swollen from crying."
"I just want him executed ... I even asked the police if I could hit him, if I could beat him," she added.
If convicted of murder, Carman could face the death penalty.
Source: BBC, Maia Davies and Helen Livingstone, June 29, 2026
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but by the punishments that the good have inflicted."
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