The words "I'm a slutty moron" appeared briefly alongside a picture of a smiling Yingluck, followed by "I know that I am the worst Prime Minister ever in Thailand history!!!". It was signed by "Unlimited Hack Team".
The government quickly shut down the website and warned the perpetrators they faced tough punishment if caught.
"It might have been done by some teenagers or maybe it was for political purposes," said the prime minister's secretary-general, Suranand Vejjajiva.
"Hacking a website is easy but don't forget that checking who did it is not hard either," he told reporters.
"If we find them, they will be charged under the computer crime act," he said, referring to a contentious law that rights groups say is sometimes used to muzzle online dissent.
The incident comes days after Yingluck filed a defamation case against a cartoonist for allegedly comparing her to a prostitute on his Facebook page.
Chai Rachawat, a cartoonist for a prominent daily newspaper, posted pictures of Yingluck accompanied by the words " a prostitute is not an evil person, the hooker only sells body. But an evil woman sells the nation".
His post apparently took aim at the premier's appearance at a democracy forum a day earlier in Mongolia where she gave an unusually fiery speech condemning the 2006 coup that toppled her brother Thaksin as premier.
Since then debate has flared over what is an acceptable level of criticism of the premier, who won a landslide election victory in 2011, becoming Thailand's first female premier.
The kingdom has been riven by political divisions since the 2006 coup, with a series of rival street protests.
About 90 people were killed and nearly 1,900 were wounded during mass pro-Thaksin rallies in Bangkok in 2010, mostly during a military crackdown.