Spokesman Lt Gen Sansern Kaewkamnerd said the reports originated on a website named toptenthailand, had been shared on social media and he urged people not to share it further.
Those who shared that news would be deemed to have violated a computer crime act, he said.
Sansern said the Royal Irrigation Department and the Electricity Generating Authority of Thailand had stopped releasing water from the Bhumibol and Sirikit dams on October 5.
They still had room to receive more water, he explained, so there was no reason from them to release water as speculated.
Flash flooding in Bangkok on Saturday morning was caused by local rainfall, he said, and authorities were working around the clock to draw floodwater away from the city.