The bitterly-divided kingdom has been ruled by a junta for two years since Yingluck Shinawatra's government was booted from office.
The majority 'yes' vote in support of the charter was the first test of public opinion since 2014 coup, although independent campaigning and open debate was stifled ahead of the polls.
The vote lends legitimacy to a junta that says it alone can stabilise Thailand and detoxify politics after a decade of turmoil.
Unofficial results released by the Election Commission showed 61.4 per cent of the country backed the document, with 38.6 per cent voting 'no'.
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The turnout was just under 55 per cent -- a low number in part due to the ban on campaining.
"I accept the decision of the people," Yingluck said in a social media post, in her first reaction to yesterday's poll, adding she was "not surprised" at the result due to the bar on debating the document.
Yesterday marked her family's first loss at the polls since a charter referendum in the wake of a 2006 coup that toppled Thaksin Shinawatra -- Yingluck's billionaire elder brother who sits at the crux of Thailand's political schism.
The Shinawatra clan have won all general elections since 2001, scooping up votes by promising greater wealth and opportunity to the nation's poor, especially in the north and northeast.
Yingluck's pro-democracy Peau Thai party had been widely assumed to still enjoy mass support and maintain the power to mobilise their voters.
Experts said the vote in part reflected the impact of the junta's ban on campaigning.