Entire neighbourhoods were inundated and the drainage network swamped after 320 millimetres of rain fell during two or three days last week, or nearly as much as the sprawling low-lying city usually gets in all of September.
"Over 80,000 residents living in flooded areas are unable to leave their homes," Unsit Sampuntharat, the permanent secretary of the interior ministry, said on Monday, adding that mobile kitchen and boats were being mobilised in relief efforts.
"The goal is to provide three meals a day to all residents throughout the ... flooding period and for at least another seven after waters recede," he added, amid intermittent showers on Monday.
Bangkok is among 41 provinces across Thailand hit by floods since September 16 that killed at least eight people and disrupted more than 580,000 homes, the ministry said.
The floods could cause economic damage of at least 11 billion baht ($A466 million) nationwide, with the losses likely to rise unless the situation improved, said Aat Pisanwanich of Rangsit University's economics department.
Residents waded through waist-deep waters in the capital's hard-hit neighbourhood of Bang Kapi to buy essentials from vendors, some of whom set up stalls with floating plastic boxes and drums, three days after incessant rain brought floods.
At least 5901 people in Bangkok had moved to 234 evacuation centres, a city administration official said.
Sitting on a river delta of soft clay just 1.5 metres above sea level, heavily built up Bangkok is sinking by several millimetres a year, while its population of more than 8.4 million often battles floods in its six-month-long rainy season.
Further east, in the coastal province of Rayong abutting a major industrial belt, authorities ordered evacuations on Monday after flood waters swelled two reservoirs beyond their capacity.
Catastrophic southern floods in 2025, triggered by 630mm of rain over three days, killed at least 145, inundated more than 33,000 homes and damaged critical facilities such as schools, hospitals and highways.
