BACKGROUND: The prevalence of diabetes has increased dramatically in the last decade. Compounding the problem are undiagnosed cases of type 2 diabetes mellitus. These respondents are those who do not know that they have the disease. Undiagnosed cases have substantial implications as they are at more risk to develope fatal complications. This study aims to determine the prevalence of undiagnosed T2DM and to identify its associated factors in Malaysia. METHODS: A nationwide cross-sectional study was conducted involving 19,935 respondents. Two-stage stratified sampling design was used to select a representative sample of the Malaysian adult population. Face-to-face interviews using structured, validated questionnaires were used to obtain data from the respondents. Respondents who claimed that they were not diagnosed with diabetes before were asked to undergo a finger-prick test. RESULTS: The overall prevalence of T2DM was 17.5% while the prevalence of undiagnosed T2DM was 9.2% (n=2103). Respondents aged 60 years old & above had the highest percentage of undiagnosed T2DM at about 13.6 %, followed by those with no formal educational at 12.9%, among Indians were 11.9%, among female at 9.2%, among non-working citizen at 9.8%, widowed/divorced (12.0%), smokers (9.5%), obesity (13.6%) and hypertensive (12.8%). Multivariable analyses revealed that age group, ethnicity, education level, marital status, obesity and hypertensive were more likely to have undiagnosed T2DM. CONCLUSION: This study showed a high prevalence of undiagnosed T2DM in Malaysia. Factors associated with undiagnosed diabetes mellitus were obesity, age, ethnicity, educational level and hypertension. Screening is essential among adults aged 30 to 60 year old to enable early intervention and prevent development of serious complications of this disease.