Background: Probation officers tend to hold stigmatising attitudes towards individuals on their caseload with mental illnesses. This can result in poor outcomes for justice-involved people with mental illnesses. Though anti-stigma interventions in the general public and among other criminal-legal professionals hold promise, these interventions have not been tested among probation officers. Aims: To measure levels of stigma towards mental illnesses among probation officers before and after a brief online training related to mental illness. Methods: Two hundred and seventy-five probation and parole officers in one Midwest state completed a three-module online training programme designed to increase knowledge and skills for working with clients with mental illnesses. Officers completed a pre-and post-training assessment of level of stigmatising attitudes towards individuals with mental illnesses. Results: Officer levels of stigma statistically significantly decreased from pre-test (8.22) to post-test (7.37; p < 0.001, t = 6.13). Nevertheless, pre-training stigma scores were the strongest predictor of stigma levels at post-test (p < 0.001, b = 0.90).Conclusions: While basic training can significantly decrease mental disorder stigmatising scores for probation officers, the practical effect of this brief online experience was very small. Nevertheless, the fact that such brief and generalised input can have impact at all is encouraging. Implementation