Communications Minister Sitthichai Pookaiyaudom said the site was banned after a 44-second film showing graffiti over the king's face was aired.
King Bhumibol Adulyadej, 79, is revered and it is forbidden to insult him.
The minister said a ban came after the Thai government asked YouTube's owner Google to remove it and they declined.
The contentious film also shows feet being placed over the king's face - an offensive act to Thais, who consider feet dirty.
"It's a serious case of lese majeste," said Mr Sitthichai, referring to crimes of offending the country's monarchy. "We asked Google to remove it some days ago, but they refused to."
A week ago, I posted about a Swiss guy getting 10 years for drunkenly defacing portraits of the venerated monach.