The unusually severe dust storm has blocked out the Sun over one quarter of the Red Planet, blanketing an area spanning 14 million square miles (35 million square kilometers), NASA said.
Opportunity, located in a spot called Perseverance Valley, "has fallen asleep and is waiting out the storm," said John Callas, Opportunity project manager at NASA's Jet Propulsion Laboratory.
"We are concerned but we are hopeful that the storm will clear and the rover will be able to communicate with us."
The storm was first detected on May 30, and grew worse in recent days.
The robotic vehicle -- one of two currently operating on Mars -- has shut everything down except its master clock, and last communicated with Earth on June 10.