BACKGROUND: Hamstring injuries are the most common muscle injury in male amateur soccer players and have a high rate of recurrence, often despite extensive treatment and long rehabilitation periods. Eccentric strength and flexibility are recognised as important modifiable risk factors, which have led to the development of eccentric hamstring exercises, such as the Nordic hamstring exercise. As the effectiveness of the Nordic hamstring exercise in reducing hamstring injuries has never been investigated in amateur soccer players, the aim of this study is to investigate the effect of this exercise on the incidence and severity of hamstring injuries in male amateur soccer players. An additional aim is to determine whether flexibility is associated with hamstring injuries. STUDY DESIGN: Cluster-randomised controlled trial with soccer teams as the unit of cluster. METHODS: Dutch male amateur soccer players, aged 18-40 years, were allocated to an intervention or control group. Both study groups continued regular soccer training during 2013, but the intervention group additionally performed the Nordic hamstring exercise (25 sessions over 13 weeks). Primary outcomes are the incidence of initial and recurrent hamstring injury and injury severity. Secondary outcomes are hamstring-and-lower-back flexibility and compliance. Compliance to the intervention protocol was also monitored. DISCUSSION: Eccentric hamstring strength exercises are hypothesised to reduce the incidence of hamstring injury among male amateur soccer players by 70%. The prevention of such injuries will be beneficial to soccer players, clubs, football associations, health insurance companies and society. TRIAL REGISTRATION: NTR3664.