Background: Mild traumatic brain injury (mTBI) is an emerging public health issue in high-contact sports. Understanding the incidence along with the risk and protective factors of mTBI in high-contact sports such as rugby is paramount if appropriate preventive strategies are to be developed. Purpose: To estimate the incidence and identify the risk and protective factors of mTBI in Australian nonprofessional rugby players. Study Design: Cohort study; Level of evidence, 2. Methods: A cohort of 3207 male nonprofessional rugby players from Sydney, Australia, was recruited and followed over 1 or more playing seasons. Demographic information, history of recent concussion, and information on risk and protective factors were collected. The incidence of mTBI was estimated and the putative risk and protective factors were modeled in relation to mTBI. Results: The incidence of mTBI was 7.97 per 1000 player game hours, with 313 players (9.8%) sustaining 1 or more mTBIs during the study. Players who reported always wearing protective headgear during games were at a reduced risk (incident rate ratio [IRR], 0.57; 95% confidence interval [CI], 0.40-0.82) of sustaining an mTBI. In contrast, the likelihood of mTBI was almost 2 times higher among players who reported having sustained either 1 (IRR, 1.75; 95% CI, 1.11-2.76) or more mTBIs (IRR, 1.65; 95% CI, 1.11-2.45) within the 12 months before recruitment. Conclusion: Nonprofessional rugby has a high incidence of mTBI, with the absence of headgear and a recent history of mTBI associated with an increased risk of subsequent mTBI. These findings highlight that both use of headgear and the management of prior concussion would likely be beneficial in reducing the likelihood of mTBI among nonprofessional rugby players, who compose more than 99% of rugby union players in Australia.