The message also joked about the prime minister's intelligence and sexual practices, the Bangkok Post reported. It was signed "Unlimited Hack," although the well-known group has denied perpetrating this particular effort.
The words attributed to Yingluck were in smaller letters: "I know that I am the worst Prime Minister ever in Thailand history!!!"
The website was taken down early Wednesday and was still down at the end of the business day.
Suranand Vejjajiva, Yingluck's secretary-general, said investigators had no idea yet of the motive for the hacking.
"Hacking a website is easy ... but don't forget that checking who did it is not hard either," he said. "If we find them, they will be charged under the Computer Crime Act."
Unlimited Hack Team, which has carried out actions aimed at government and news sites, posted a message on its Facebook page saying it always takes responsibility for its actions.