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Brucellosis is a highly contagious zoonotic disease with significant economic consequences. In the last 50 years, no brucellosis outbreak has been documented in Sierra Leone.  In March 2021, Koinadugu District reported suspected cases of brucellosis in cattle. The study was aimed to confirm the diagnosis of disease, determine the magnitude of the outbreak, identify the sources of infection, and institute control measures. Brucellosis suspected case wasany animal in the Koinadugu District with abortions, retention of placenta, stillbirth, the birth of weak offspring, or swelling of testicles, from February 21 to March 21, 2021. A confirmed case was any suspected case and laboratory positive for brucellosis. We interviewed livestock officials and animal owners, and reviewed clinical records to collect animals' clinical characteristics, travel history, and immunization status. Blood samples from animals with suspected brucellosis were collected for laboratory confirmation by Rose Bengal Test (RBT). We conducted active case searches at veterinary clinics and in the affected communities to identify additional cases and assessed the animal flock's environmental condition. Four Hundred (400) animals with suspected brucellosis had been reported, of which 150 were confirmed positive by RBT. All confirmed cases were female, where 135 (90%) between 1 and 2 years, and 15 (10%) were Heifers (12-14 months). Of the 150 confirmed cases, 100 (66%) were pregnant, among them, 25 (25%) were aborted. Almost half of the cases presented with vaginal discharge, retained placenta, and uterine infection. Three out of 50 cattle imported from Guinea died before being tested for brucellosis and no animals were vaccinated against brucellosis. No additional animal or human cases were reported.  We observed that fetuses, aborted materials, and sick animal carcasses were left in open places for days before being disposed. Brucellosis outbreak was confirmed in Koinadugu District. Lack of vaccination might be responsible for the high infection rate. The source of the outbreak could be the introduction of infected animals from other farms. We distributed personal protective equipment and educated farmers about the safe disposing of aborted fetuses and placentas. We recommended enforcing the existing animal import regulations and also engaging in a nationwide vaccination campaign.