The Thai navy began removing a floating house off the coast of the southern island of Phuket belonging to a US-Thai couple.
The effort will take half a day, with the structure expected to be moved ashore in the afternoon, according to the navy.
Seasteading refers to building floating societies outside the territory claimed by any government.
The structure was built by Chad Elwartowski, a 46-year-old American, and his partner Supranee Thepdet, also known as Nadia Summergirl.
The couple, who invited people to join them seasteading in a video posted online last month, has been accused of violating Thailand's sovereignty after the erection about 26km off Phuket.
They have insisted they did not violate Thailand's laws as the cabin is located in international waters, but Thai authorities say it lies within a 321km exclusive economic zone.
The couple has not yet been formally charged over the structure, police said. If prosecuted, they could face the death penalty or life imprisonment.
The American man's Thai visa has also been revoked, according to the navy.
Currently in hiding, the pair said they are safe and not a threat to national security.
"Happy Easter everyone. Nadia and I are still safe," Chad Elwartowski wrote on his Facebook page late Sunday.
"I am a pacifist and Nadia is a Buddhist and Buddhists are supposed to respect the sanctity of life. So there is no way we could ever be a threat to anyone," he added. "We just wanted to be free."
