The health committee of Bayan Nur issued a third-level alert, the second lowest in a four-level system.
The alert forbids the hunting and eating of animals that could carry plague.
It also asks the public to report any suspected cases of plague or fever with no clear causes, and to report any sick or dead marmots.
The warning comes a day after a hospital in the Inner Mongolian city reported a case of suspected bubonic plague on Saturday.
It follows four reported cases of plague in people from Inner Mongolia last November, including two of pneumonic plague, a deadlier variant of plague.
The bubonic plague, known as the "Black Death" in the Middle Ages, is a highly infectious and often fatal disease that is spread mostly by rodents.
Plague cases are not uncommon in China, but outbreaks have become increasingly rare.
From 2009 to 2018, China reported 26 cases and 11 deaths.