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Between December 2016 and April 2017, two cases of invasive Group A Streptococcus (GAS) infections were reported at a Canadian military training facility. An outbreak was declared and a field investigation was launched to characterize the outbreak and identify associated risk factors to limit transmission. Throat culture data from military personnel at the garrison were analyzed. Investigators tracked invasive GAS cases and non-invasive hospitalized GAS cases, and conducted site visits and case interviews. Sensitivity and specificity for a rapid antigen detection test were evaluated. Molecular typing and phylogenomic relationships of outbreak isolates were analyzed using whole-genome sequencing. During this outbreak, four invasive cases were reported and six non-invasive cases were hospitalized. In a sample of 705 throat cultures examined, 35.2% were GAS-positive. Among 65 platoon contacts of one invasive case, 30.2% were GAS-positive. Reluctance to seek medical care, challenges in following cough etiquette, and low compliance with antibiotics were identified among recruits. The rapid antigen detection test had low sensitivity (31.6%) during the outbreak. The outbreak sequence type was emm6.4 and outbreak isolates were highly related phylogenetically, differing by 0-4 single nucleotide variants. This is the first report of a GAS outbreak among Canadian military trainees. Increased surveillance of GAS infections, increased control measures and outbreak-specific clinical guidelines were implemented in-garrison. No further invasive GAS cases were identified. A GAS surveillance system was implemented and efforts to improve antibiotic compliance and medical consultation were recommended.