Twenty-one people from Bangkok's Sathon area have tested positive for the Zika virus, including a woman who has recently given birth.
The woman delivered a healthy baby at 37 weeks, the Bangkok Metropolitan Administration Health Department said. Her name was not released.
She and the baby have been placed under close watch by the health authorities.
The department said the virus may have spread from her husband who had spent time in Singapore, which reported more than 240 Zika cases since the first locally infected case was detected on Aug 27.
Earlier, another woman who was seven months pregnant was reported to have contracted the virus late into her pregnancy.
The woman was not confirmed to have tested positive to the virus.
She is in a safe condition, the department said.
The department said there have been no reports of anyone dying from the Zika virus in Thailand.