ABSTRACT Middle East respiratory syndrome (MERS) is a highly lethal pulmonary infection. Serum from convalescent MERS patients may provide some benefit but is not readily available. In contrast, nearly all camels in the Middle East have been infected with MERS-CoV. Here, we show that sera obtained from MERS-immune camels augment the kinetics of MERS-CoV clearance and reduce the severity of pathological changes in infected lungs, with efficacy proportional to the titer of MERS-CoV-neutralizing serum antibody. IMPORTANCE Middle East respiratory syndrome, caused by a coronavirus, is highly lethal, with a case fatality rate of 35 to 40%. No specific therapy is available, and care is generally supportive. One promising approach is passive administration of sera from convalescent human MERS patients or other animals to exposed or infected patients. The vast majority of, if not all, camels in the Middle East have been infected with MERS-CoV, and some contain high titers of antibody to the virus. Here, we show that this antibody is protective if delivered either prophylactically or therapeutically to mice infected with MERS-CoV, indicating that this may be a useful intervention in infected patients.