Farmers in the world's second-biggest rice exporter usually plant their main crop in May, the beginning of the rainy season, for harvest between August and October.
But this year, the rain has been sparse and drought has been declared in more than a dozen provinces in northern and northeastern rice regions.
The government is considering measures such as cloud seeding to try to bring rain but in the meantime, farmers have been asked to hold off.
"We would like to ask farmers not to grow new crops of rice because there may not be enough water," Irrigation Department official Sanya Sangpumpong told Reuters.
The pumping of water to keep crops alive had led to a serious depletion of reservoirs, he said.
"Human consumption must be prioritised first," he said.
The biggest impact would be on jasmine rice, which is planted in August for harvest by the end of the year. It is grown largely in the northeast.
Rainfall in the main rice-growing regions was the lowest in 10 years, at 12% below average, the Meteorological Department said. Rain in August and November was expected to be 5% to 10% below average.
A rice farmer in the northeastern province of Khon Kaen said it was the worst dry spell he had seen in years and he had been pumping water from a reservoir to keep his first crop alive.
"If it does not rain then I won't plant a new crop," Pradit Sirithammajak, 48, told Reuters.
"It's not worth the cost."
At the same time, the level of water in the Mekong River, which passes northern and northeastern Thailand, had fallen below a historic low seen in 1992, according to the inter-governmental Mekong River Commission.
This was partly due to the low rain but also because China was holding back more of the river's water in a hydroelectric dam on its upper reaches. - Reuters