The flooding, which was roof-high in some areas, has affected nearly one million people in ten southern provinces since it started a week ago, the Interior Ministry said in a statement.
At least 18 people have died and one is missing, it added, with the rains turning roads into rivers, inundating farmland and damaging more than 1,500 schools in the region.
The downpour is expected to persist for at least two more days, according to Thailand's Meteorological Department, which warned of flash floods.
"The situation is very bad today and tomorrow. It's still raining heavily," said Junjuda Pornsri, a meterological official.
Military bases across the region have been mobilised to help evacuate flood victims, provide temporary shelters and distribute emergency aid, the government said Saturday.
In hard-hit Nakhon Si Thammarat province, two army helicopters were deployed to airlift food to families trapped inside their homes in Cha-uat district.
Bapha Suthiphanya, a 60-year-old who has spent the past three nights in a makeshift government shelter in the district, said she was forced to evacuate her home after the waters rose above her head.
"I was so shocked and scared. I've never seen water like this and I also can not swim," she told AFP.
- Peak tourist season -
The monsoon rains are unusually heavy for this time of year in Thailand, which normally sees a three-month stretch of relatively dry and cool weather starting in November.
It is high season for tourists who flock to the kingdom's island resorts, powering a crucial sector of the economy.
But the deluge has already disrupted beach holidays in several traveller hotspots, including the popular islands of Samui and Phangan.
Hundreds of tourists have had their flights delayed, while train and bus services on the mainland have also been suspended.
Yet some travellers are refusing to let the storm stop the fun, with photos doing the rounds on social media of tourists coasting through flooded streets on pool floats, sipping drinks.
"Some tourists are enjoying the flooding, they're taking pictures and going swimming," said Nongyao Jirundorn, a tourism official on Samui island.
Neighbouring Malaysia was also hit by severe flooding earlier this week, with thousands stranded in relief centres in two northeastern states.
But by Saturday, the number of evacuees in Kelantan and Terengganu had dropped to about 13,500, from almost 23,000 Wednesday, as weather conditions improved and authorities forecast less rainfall over the weekend.
Prime Minister Najib Razak visited Kelantan on Saturday and met with people seeking shelter at a relief centre.