Background: Hypertension is an important cause of morbidity and mortality worldwide which often remains undiagnosed and subsequently untreated. Data from diverse populations on the prevalence of undiagnosed hypertension therefore become imperative. This study examined the prevalence and risk factors of undiagnosed hypertension among traders at the in a major regional market in Nigeria. Subjects and Methods: Blood pressure measurement, body mass index (BMI) and socio-demographic data of 411 traders at the Monday market in Northeastern Nigeria were obtained in this longitudinal study. Hypertension was defined as systolic blood pressure of 140 mmHg or higher, or diastolic blood pressure of 90 mmHg or higher in two consecutive measurements. Inferential statistics was used to examine associations between prevalence of undiagnosed hypertension and socio-demographic characteristics and BMI of the traders. Results: Mean (SD) age of the traders was 36.3 (10.4) years while male traders were in the majority (83%). Undiagnosed hypertension was detected in 25% (102/411) of the traders. Prevalence of undiagnosed hypertension was significantly associated with the age (P<0.001), marital status (P<0.001) and body mass index (P=0.03) of the traders with higher prevalence among older, married and obese traders. Conclusion: One in four market traders had undiagnosed hypertension with a significantly higher prevalence among older, married and obese traders. The need for regular high blood pressure screening and effective preventive and attenuating strategies is emphasized.