Background: Self-monitoring of blood pressure (BP) among hypertensive patients is an important aspect of the management and prevention of complication related to hypertension. However, self-monitoring of BP among hypertensive patients on scheduled follow-up in hospitals in Ethiopia is unknown. The aim of the study was to assess knowledge and attitude of self-monitoring of BP among adult hypertensive patients. Methods: A cross-sectional survey was conducted on 400 adult hypertensive patients attending follow-up clinics at four public hospitals of Arsi Zone, Oromia Regional State, Ethiopia. The data were collected from patients from March 10, 2019 to April 8, 2019 by face-to-face interview using a pretested questionnaire and augmented by a retrospective patients' medical records review. The data were analyzed using the SPSS version 21.0 software. Results: A total of 400 patients were enrolled into the study with the response rate of 97.6%. The median age of the participants was 49 years (range 23-90 years). More than half (225 [56.3%]) were male. The majority (160 [40%]) were married and more than two-thirds (282 [70.5%]) were Oromo by ethnic background. About 206 (51.5%) had attended primary education. The proportion of patient's knowledge toward self-monitoring of BP and the practice of self-monitoring of BP among hypertensive patients was 31.5% (n=126 [95% CI; 26.5, 36.5]) and 7.75% (n=31 [95% CI; 5.3, 10.5]) respectively. The multivariable logistic regression analysis revealed; higher education (AOR=2.73, 95% CI [1.33, 13.88)], governmental employed (AOR=1.52, 95% CI [1.06, 6.48]), having an income of >3500 Ethiopian Birr (AOR=2.16, 95% CI [1.56, 7.39]), duration of hypertension >6 years (AOR=1.87, 95% CI [1.21, 6.37]), having health insurance (AOR=3.56, 95% CI [1.39, 10.53]), having co-morbidities (AOR=3.93, 95% CI [1.35, 10.32]), receiving a health professional recommendation toward self-monitoring of BP (AOR=6.08, 95% CI [2.45, 15.06]), and having an awareness of hypertension-related complication (AOR=3.94, 95% CI [1.34, 11.44]) were factors significantly associated with self-monitoring of BP. Conclusion: In this study, the proportion of knowledge of self-monitoring of BP and the practice of self-monitoring of BP among hypertensive patients on follow-up were low. Educational programs on self-monitoring of BP including teaching through demonstration may be needed to be in place.