The fire has affected at least 800,000 square metres (200 acres) of forest across three hills in Nakhon Nayok province, to the northeast of the Thai capital, with two districts declared emergency zones.
The blaze comes as Thailand grapples with a spike in pollution caused in part by agricultural burning that has seen the air in some northern cities rank among the dirtiest and most hazardous in the world this week.
Since the start of the year 1,730,000 people have needed hospital treatment for respiratory conditions caused by air pollution, according to the latest figures from the health ministry.
The fire is not threatening any major population centres but it is close to the edge of the Khao Yai National Park, the kingdom's oldest national park and part of a UNESCO-listed forest complex stretching to the Cambodian border.
Thai Prime Minister Prayut Chan-O-Cha has dispatched his interior minister to coordinate operations on the ground, his office said in a statement.
"PM Prayut and minister of defence have closely monitored the wildfire situation in Nakon Nayok and ordered relevant organisations to help putting out the fire urgently," government spokesman Anucha Burapachaisri said.
Prayut, who faces a general election on May 14, on Thursday urged the top official from regional bloc ASEAN to help coordinate efforts to reduce cross-border pollution caused by agricultural burning.
Farmers across Southeast Asia burn off stubble in fields every year after harvest time, causing widespread air pollution.
Thailand's north is particularly affected and this week the cities of Chiang Mai and Chiang Rai topped monitoring firm IQAir's worst air quality list -- beating the likes of Delhi and Beijing.
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