Background: Dietary diversity is commonly used as an indicator of diet quality and nutritional adequacy, yet its relationship with non-communicable diseases (NCDs) remains inconsistent across populations. Understanding dietary patterns in relation to NCDs is essential for developing effective preventive strategies in clinical and public health settings. Objectives: To assess dietary diversity patterns and examine their association with selected non-communicable diseases among adult outpatients attending a tertiary care hospital in Tiruppur, Tamil Nadu. Methods: A hospital-based cross-sectional analytical study was conducted among 1,054 adult outpatients. Data on socio-demographic characteristics, lifestyle factors, and dietary habits prior to NCD diagnosis were collected using a pre-tested questionnaire. Dietary diversity score (DDS) was calculated based on consumption frequencies of eight food groups and categorized as low, intermediate, or high. Associations between food group consumption, DDS categories, and NCD prevalence were analyzed using chi-square tests and multivariable logistic regression. Results: Most participants had an intermediate DDS. No statistically significant association was observed between DDS categories and the prevalence of diabetes mellitus, hypertension, heart disease, cancer, or their common co morbidities. However, significant associations were identified between specific food group consumption frequencies and NCD outcomes, particularly diabetes mellitus and hypertension. Weekly consumption of aerated drinks and fried foods showed higher odds of hypertension, while certain animal-source foods were associated with higher odds of diabetes. Conclusion: Overall dietary diversity alone was not significantly associated with NCD prevalence, whereas specific dietary components demonstrated meaningful associations. Nutritional interventions should prioritize diet quality alongside diversity to support NCD prevention.