Eleven people in Bung Na Rang district of Phichit province are confirmed to be infected with the zika virus while 27 others are being monitored for confirmation, provincial governor Wirasak Wichitsaengsi said on Tuesday morning.
Laboratory test results that became available on July 1 had confirmed the 11 infections.
Mr Wirasak said 27 other people believed infected with the mosquito-borne virus were also being tested.
No names were released or further information about the infections, other than all the patients were being treated in hospital.
The detected infections prompted the local health office to send a mosquito eradication team to spray 667 houses in the affected areas, he said.
The governor rejected rumours that a paramedic in Phichit had recently died of zika. An investigation was underway to determine the exact cause of death.
There was no reason to panic, Phichit was not the only province where zika cases were reported, he said. Chaiyaphum, Ubon Ratchathani and Nong Khai had previously reported a number of cases.
Last September, the Public Health Ministry confirmed that two children born in Thailand with abnormally small heads were infected with the zika virus.
The ministry said these were the first recorded cases of babies being infected by the virus in Thailand and diagnosed with zika-linked microcephaly.
Zika infection during pregnancy can cause a birth defect of the brain called microcephaly and other severe brain defects, according to information published on the website of the US Centre for Disease Control and Prevention (CDC).
It is also linked to other problems, such as miscarriage and stillbirth. There have also been increased reports of Guillain-Barré syndrome, an uncommon sickness of the nervous system, in areas affected by zika.
The most common symptoms of zika are fever, rash, headache, joint pain, red eyes and muscle pain
Symptoms can last for several days to a week. People usually do not get sick enough to go to the hospital, and they very rarely die of zika. Once a person has been infected and recovers, they are likely to be immune to future infection.