The new cases raised the accumulated total in Thailand since the start of the outbreak to 4,237. No new deaths were reported.
Taweesilp Visanuyothin, the Centre for Covid-19 Situation Administration (CCSA) spokesman, said six of the new cases were Thais who returned from Tachilek to Mae Sai district of Chiang Rai through an immigration checkpoint on Friday.
Most of them had worked at entertainment places in the Myanmar border township, he added.
All were quarantined.
Hundreds of Thais worked at nightspots in Tachilek before the town was locked down due to the virus outbreak.
Dr Taweeslip called on all Thais, including those who sneaked into Myanmar, to return home through a border checkpoint so the government can contain Covid-19.
"We will not send you to jail. We will send you to state quarantine facilities," he said.
More than 260 Thais working in Tachilek have come back through the checkpoint so far.
Aside from the six cases from Tachilek, 17 other Thai citizens tested positive for the novel coronavirus after flying back into the country from overseas. They had arrived from the United Kingdom, France, Azerbaijan, Germany, Ukraine, Poland and Bahrain.
In addition to the new Thai patients recorded on Monday, four foreigners were found infected after travelling to the country from the UK, Indonesia, Sweden and Jordan.
The single local transmission was a health worker who caught the virus from a patient.
