The U.S. space agency said the storm, which is located in the southern hemisphere of Mars, was spotted earlier this month by the Mars Reconnaissance Orbiter.
The storm came within 1,350 km of the Opportunity rover, but it did not damage any equipment. On the other side of the planet, the Curiosity rover has identified changes in air pressure and temperatures at night.
Opportunity runs on solar energy. If the current storm continues to grow, the rover could lose some of its power.