The death occurred inside a quarantined part of Gobi-Altai province in western Mongolia, health officials said. Two other teenagers also ate the marmot.
Officials say more than a dozen people who came in contact with the victim, including the other two teenagers, have been quarantined and given antibiotics.
Mongolia is currently grappling with an outbreak of the plague thought to be spreading through marmots, a large rodent that is sometimes hunted for food. Officials in Mongolia and neighbouring Russia have urged people not to hunt or eat marmots during the outbreak.
Two other Mongolians were recently diagnosed with the plague in an unrelated incident, and another case was reported in the Chinese region of Inner Mongolia earlier this month.
The plague is caused by a bacteria called Yersinia pestis, and a handful of cases crop up in different parts of the world every year. In the U.S., for example, a squirrel tested positive for the disease on Saturday.
The disease causes flu-like symptoms such as fever, chills, headache, nausea and swollen lymph nodes within a week of exposure. About 50-60 per cent of all untreated cases are fatal, according to the World Health Organization.
The bubonic plague is one of three forms of plague caused by the bacteria, which once wreaked havoc on the world as the Black Death. The current COVID-19 pandemic pales in comparison to that earlier plague, which killed an estimated 50 million people from 1347 to 1352.
The World Health Organization says the plague is no longer a global threat because it can be treated with antibiotics if diagnosed early enough. However, it’s also unlikely to ever disappear, and a few thousand cases are diagnosed around the world each year.
—With files from The Associated Press