The boy caught the rare bacterial illness after hunting and eating a marmot, according to the health ministry.
"We quarantined the first 15 people who came into contact with the boy," who lived in the Gobi-Altai region of the country. "Those 15 people are receiving antibiotic treatment," Narangerel Dorj, head of public relations at the health ministry, told reporters.
Five districts of the province were placed under six-day quarantine.
Earlier this month two other cases of bubonic plague were registered in Khovd province. More than 140 people were tested, but no other cases were found.
A herdsman also contracted the plague in China's northern Inner Mongolia region, prompting local authorities to ban the hunting and eating of animals that could carry the plague until the end of the year.
Marmot kidney
At least one person dies of the plague every year in Mongolia despite government campaigns to discourage people from eating marmot or approaching the animal.
But many in rural areas grow up learning to hunt and eat the large ground quirrel, and some believe eating its innards is good for health.
An ethnic Kazakh couple died of the plague last year after eating raw marmot kidney.
The Moscow Times reports that the recent cases prompted Russia's eastern Siberian region of Burytia to test rodents for the bubonic plague and urge residents not to hunt or eat marmot.
It also comes as Mongolia's borders remain closed due to the coronavirus pandemic.
Antibiotics
Meanwhile, the World Health Organisation says that the Plague has an incubation period of between 3 and 7 days, but can easily be treated.
"Although plague has been responsible for widespread pandemics throughout history, including the so-called Black Death that caused over 50 million deaths in Europe during the fourteenth century, today it can be easily treated with antibiotics and the use of standard preventative measures," the WHO says on its website.
