Already under public pressure to resign, which forced him to impose a state of emergency in the capital Bangkok last week, Samak was told by the court he violated the country's supreme law by hosting the TV show while being prime minister, the Thai News Agency reported. Samak had claimed he appeared on the show without taking any remuneration.
A BBC report said Samak was a TV chef before becoming prime minister.
After the unanimous court ruling, lawmakers from Samak's People Power Party agreed to nominate him to his current post in the upcoming parliament session, the TNA report said quoting a party spokesman.
CNN reported Samak was already facing charges of corruption as well an election commission accusation last week that his party had committed electoral fraud in the December elections.
Opponents of his government have asserted his government is only a proxy for ousted Prime Minister Thaksin Shinawatra. Samak had denied the allegations and refused to quit.