Police tracked down 32-year-old teacher Christopher Paul Neil in north-east Thailand. They suspect him of appearing in 200 online images of child abuse.
Mr Neil, who faces 20 years in jail if convicted, was brought to a news briefing, but did not answer questions.
Mr Neil was picked up by officers on Friday morning in the city of Nakhon Ratchasima, 250 km (150 miles) north-east of the capital, Bangkok.
He was driven to the Thai capital where he was paraded in handcuffs in front of gathered journalists at a police station.
The BBC's Chris Hogg, in Bangkok, said there were chaotic scenes as Mr Neil was led into the building, with police having to fend off a scrum of reporters and photographers.
Police chiefs said he was suspected of abusing at least 12 youngsters - at least one said to be under 10 years old.
The arrest follows lengthy efforts to identify a man seen in more than 200 online images of alleged sexual abuse involving young boys.
Officers now believe the Canadian might have abused boys in Thailand as well as in Vietnam and Cambodia - and he could face charges in any of those countries.
He could also be charged in Canada, which has laws allowing for the prosecution of its nationals for child-sex crimes committed abroad.
Thai police have appealed for more victims to come forward and indicated he would be charged in Thailand, but they did not rule out the possibility of extradition.