The fines were imposed by the industry regulator, the National Broadcasting and Telecommunications Commission (NBTC), the Daily News reported.
It quoted Col Natee Sukolrat, the NBTC deputy chairman, as saying that Dexter violated Article 37 of the 2008 Broadcasting Act. The section prohibits airing content that could "lead to the overthrow of democratic regime with the King as Head of State, or affect the national security, peace and order, and morals of the people, and actions that tend to be pornographic or lead to degradation of mind or health of the public".
Dexter tells the story of a forensic science investigator who moonlights as a murderous vigilante. It concluded an eight-season run last September on the US cable network Showtime, and has been nominated four times for Primetime Emmy awards for Best Drama Series.
The regulator fined the local channels GMM, CTH and TOT 50,000 baht each for airing the series. Reruns of Season 6 of Dexter recently concluded on the TrueVisions pay-TV service.
"Even though it is a foreign-made series that cannot be altered in its content, it is still not appropriate to be aired in Thailand," Col Natee said, according to Daily News.
The 11-person NBTC panel consists of five military officers, one police officer and five civilians.