Training for law enforcement on effective ways of intervening in mental health crises is limited. What is available tends to be costly for implementation, labor-intensive, and requires officers to opt-in. DEFUSE, an interactive online training program, was specifically developed to train law enforcement on mental illness and de-escalation skills. Derived from a stress inoculation framework, the curriculum provides education, skills training, and rehearsal; it is brief, cost-effective, and scalable to officers across the country. Participants were randomly assigned to either the experimental or delayed treatment control conditions. A multivariate analysis of variance yielded a significant treatment-by-repeated-measures interaction and univariate analyses confirmed improvement on all of the measures (e.g., empathy, stigma, self-efficacy, behavioral outcomes, knowledge). Replication dependent t-test analyses conducted on the control condition following completion of DEFUSE confirmed significant improvement on four of the measures and marginal significance on the fifth. Participant responses to BPAD video vignettes revealed significant differences in objective behavioral proficiency for those participants who completed the online course. DEFUSE is a powerful tool for training law enforcement on mental illness and effective strategies for intervening in mental health crises. Considerations for future study are discussed.