Background: Mental illnesses pose a significant burden worldwide. Furthermore, the treatment gap for mental disorders is large. A contributor to this treatment gap is the perceived stigma towards mental illness. Besides impeding one's help-seeking intentions, stigma also impairs persons with mental illness (PMI) in other aspects of their life. Studies have found that stigma may manifest differentially under different cultural contexts. Thus, this study seeks to elucidate the determinants of stigma towards PMI among lay public in Singapore using a qualitative approach. Methods: A total of 9 focus group discussions (FGD) were conducted with 63 participants consisting of lay public Singaporeans who were neither students or professionals in the mental health field, nor had they ever been diagnosed with a mental illness. Topics discussed during the FGD were related to the stigma of mental illness. Data collected were analyzed with inductive thematic analysis method. A codebook was derived through an iterative process, and data was coded by 4 different coders. Both coding and inter-rater analysis were performed with NVivo V.11. Results: In total, 11 themes for the determinants of stigma were identified and conceptualized into a socioecological model. The socioecological model comprised 4 levels of themes: 1) Individual level beliefs (fear towards PMI, perceiving PMI to be burdensome, dismissing mental illness as not a real condition), 2) Interpersonal influences (upbringing that instills stigma, intergroup bias, perceived inability to handle interactions with PMI), 3) Local cultural values (elitist mindset among Singaporeans, Chinese culture of "face", Islamic beliefs about spiritual possession and reaction towards PMI), and 4) Shared societal culture (negative portrayal by media, Asian values). Conclusions: The findings of this study improved our understanding of the various reasons why stigma exists in Singapore. The themes identified in this study concur with that of studies conducted overseas, as some determinants of stigma such as fear towards PMI are quite ubiquitous. Specifically, the themes elitist mindset among Singaporeans and perceived inability to handle interactions with PMI were unique to this study. It is highly recommended that future anti-stigma campaigns in Singapore should incorporate the findings of this study to ensure cultural misgivings and beliefs are addressed adequately.