Bangkok's second airport has suspended all flights after floodwaters breached its northern perimeter.
Don Muang airport, used mainly for domestic flights, is in northern Bangkok - the area of the capital worst hit by the flooding.
The international airport, in another part of the city, is still operating.
Thailand has been hit by heavy monsoon rain since July, leading to flooding which has hit swathes of the country and left more than 360 people dead.
Water from inundated central areas is now running south to the sea.
Officials have been trying to drain it to the east and west of Bangkok, but they have been forced to open sluice gates into the city because of the sheer volume of water building up outside Bangkok's flood barriers.
Seven districts of the capital are said to be at risk.
Central areas remain dry, but the Chao Phraya river, which bisects the city, reached a record high on Monday.
Don Muang airport - a hub for domestic flights, low-cost carriers and some cargo - has been threatened by encroaching floodwater for several days.
An official said the flooding was affecting perimeter areas, not the runway.
Nok Air, a budget carrier, said early on Tuesday it was suspending operations at the airport because water was flowing into an area to the north.
The airport is also now being used as an evacuation centre and as the headquarters of the government's flood relief operations. Relief officials said they had no plans to relocate.
Thai authorities have declared a holiday in several provinces, including Bangkok, to help people cope with the flooding.
Schools and offices will close from Thursday through to the end of Monday, creating a five-day break.