Introduction: Hypertension (HTN) is a silent killer, accountable for life-threatening complications. An individual's illness perception may affect adherence to treatment which is crucial to prevent complications of HTN. The objective of this study was to identify illness perception and treatment adherence among patients with HTN in a tertiary hospital in Kathmandu, Nepal. Methods: Descriptive correlational study was conducted in the outpatient department of Manmohan Cardiothoracic Vascular and Transplant Center, Kathmandu Nepal. Nonprobability purposive sampling was used. A face-to-face interview was conducted from September to December 2018, using a structured questionnaire that included sociodemographic variables, illness perception questionnaire (revised) and Hill bone compliance to high blood pressure therapy scale. Data analysis was done by using descriptive and inferential statistics (chi-square test, Spearman rank correlation). Results: Among 204 participants, 51% were male, 77% were literate, mean ± S.D. age was 60±12. About 72% experienced headache and 88% said that headache is related to HTN. Behavioural factors and psychological factors were regarded as the leading cause of HTN. Almost 63% participants believed HTN as highly threatening illness. Higher scores in timeline (acute/chronic), personal control, and treatment control revealed that patients believed HTN as a chronic disease with a higher rate of personal and treatment control. Regarding treatment adherence, the mean score was 16.58 (SD = 2.08), and only 14.7% had perfect adherence. Participants were more adherent to medication and appointment keeping rather than reduce salt intake. Duration of HTN diagnosis (p=0.027) and duration under HTN medication (p= 0.021) were found to be significantly associated with treatment adherence. There was a significant positive correlation between illness perception and treatment adherence (ρ = 0.282, p<0.01). Conclusion: Illness perception and treatment adherence are correlated. Hence, it is beneficial to improve illness perception to achieve perfect treatment adherence. Reinforcement is essential to maintain adherence to both medications and behaviour therapy.