Background: There are increasing challenges related to mental health in the public health system, and almost 80% of mental illnesses are reported in low- and middle-income countries. The stigma associated with mental illness leads to reduced access to health services, inadequate treatment, and poor outcomes. This paper, drawing on Norton1 conceptualization of language investment, investigates the effect of English language learning on self-stigma among patients suffering from mental disorders. Methods: An experimental design with a pre-post test format and a six-month follow-up with English language instruction and a control group was used for this study. A total of 52 adults with mental illness diagnoses from a center for patients suffering from mental disorders in Isfahan, Iran, participated in this study. Data were collected using an English language proficiency test and the Self-Stigma of Mental Illness Scale (SSMIS), a self-report questionnaire to assess the internalization of stigmatic views. Results: The analysis indicated that the experimental group showed significant improvements in being empowered with higher levels of English language proficiency and stigma reduction (P<0.01). The findings of this study provide theoretical support for Norton’s model of investment and demonstrate its applicability among people with mental problems. Furthermore, the study provides evidence that it is feasible to implement successful English language teaching among patients with mental illness, which has received insufficient attention in academic studies. Conclusion: Policymakers, psychiatrists, doctors, and people dealing with mental diseases can use English language instruction to reduce stigma and promote a non-stigmatizing attitude among people with mental illnesses. The current study contributes to our understanding of English language teaching in the medical field. Results of the study support the use of second-language teaching to reduce stigma among people suffering from mental illnesses.