Twenty-one vehicles crashed and Montana Highway Patrol Sgt. Jay Nelson said officials believe the weather was a factor.
"It seems as though there are solid breezes, causing no ability to see," he said.
While the Highway Patrol didn't have a prompt count of the quantity of wounds, Nelson said extra ambulances must be called from Billings to help.
Gov. Greg Gianforte said on Twitter: "I'm deeply saddened by the news of the massive accident near Hardin.
Kindly go along with me in appealing to God for the people in question and their friends and family. We thank our first responders. Thank you for your service."
Montana Attorney General Austin Knudsen, who oversees the highway patrol, said in a statement:
"The Montana Highway Patrol is on the scene with other first responders and is investigating the incident. We will release more information.
"will as soon as it becomes available and appropriate to the lives lost and their loved ones.
"My prayers are with everyone affected by the tragic events that occurred during the dust storm in Big Horn County today," Knudsen added.