Pantip is the most popular chat room in Thailand and the political pages often feature feisty debate about democracy, the military junta and the legitimacy of the coup that brought them to power last year.
The ICT's move comes just days after they banned popular video-sharing website YouTube because of clips considered offensive to the nation's revered King Bhumibol Adulyadej.
Since coming to power, the military have blacked out international news broadcasts, seized a private television station and blocked political websites.
Thailand has blocked some 45,000 websites, the group Freedom Against Censorship Thailand have said.
Most are pornographic but the government also targets sites critical of the king or supportive of Thaksin Shinawatra, the deposed prime minister.
Pantip users had recently been debating the censorship of YouTube, although it was not clear if this was the reason it was censored. The ICT were not immediately available for comment.