Rocky Mountain spotted fever (RMSF) is a bacterial disease spread by ticks. It typically begins with a fever and headache, which is followed a few days later with the development of a rash. The rash is generally made up of small spots of bleeding and starts on the wrists and ankles. Other symptoms may include muscle pains and vomiting. Long-term complications following recovery may include hearing loss or loss of part of an arm or leg.

The disease is caused by Rickettsia rickettsii, a type of bacterium that is primarily spread to humans by American dog ticks, Rocky Mountain wood ticks, and brown dog ticks. Rarely the disease is spread by blood transfusions. Diagnosis in the early stages is difficult. A number of laboratory tests can confirm the diagnosis but treatment should be begun based on symptoms. It is within a group known as spotted fever rickettsiosis, together with Rickettsia parkeri rickettsiosis, Pacific Coast tick fever, and rickettsialpox.

Treatment of RMSF is with the antibiotic doxycycline. It works best when started early and is recommended in all age groups, as well as during pregnancy. Antibiotics are not recommended for prevention. Approximately 0.5% of people who are infected die as a result. Before the discovery of tetracycline in the 1940s, more than 10% of those with RMSF died.

Fewer than 5,000 cases are reported a year in the United States, most often in June and July. It has been diagnosed throughout the contiguous United States, Western Canada, and parts of Central and South America. Rocky Mountain spotted fever was first identified in the 1800s in the Rocky Mountains.