The government reported 985 new Covid-19 cases on Monday, a new daily record since the pandemic began, bringing the accumulated total to 33,610.
All but five of the new infections were transmitted inside Thailand.
No additional deaths were reported, leaving the toll at 97.
The Centre for Covid-19 Situation Administration (CCSA) said Chiang Mai led new cases with 246 transmissions, followed by Bangkok (137) and Chon Buri (92). The Chiang Mai figures included a big jump reported by the province on Sunday afternoon.
The national CCSA report showed Chiang Mai with 57 more cases than on Sunday morning, and Bangkok with 99 fewer.
Department of Disease Control director-general Opas Karnkawinpong said the figures after the Songkran holiday will define the direction of the Thailand’s third wave of Covid-19.
"The trend is still upward. We have to look at the figures after Songkran," Dr Opas said.
He said most new cases were still linked to nightspots and he warned that the ongoing university summer break could make new cases jump if students lower their guard in personal activities and extracurricular programmes such as summer camps in remote areas.
The five imported cases logged on Monday comprised two from Russia, two from Germany and one from Sweden.