Diaphragmatic breathing, a deep breathing technique, has been reported to improve autonomic function by reducing sympathetic activity and increasing baroreflex sensitivity. This literature review aimed to (1) examine the effects of diaphragmatic breathing on physiological and psychological measures in prehypertensive or hypertensive adults and to (2) determine the appropriate length, frequency, and duration of an effective diaphragmatic breathing exercise in the management of prehypertension and hypertension. Relevant studies were searched using electronic databases, and 13 studies that met the inclusion criteria were included. The synthesis of the findings revealed that voluntary diaphragmatic deep breathing resulted in decreased of systolic and diastolic blood pressures, reduced heart rate, a relaxing effect, and reduced anxiety in hypertensive or prehypertensive individuals. It is concluded that voluntary diaphragmatic breathing at <10 or 6 breaths per minute for 10 min twice a day for 4 weeks was effective in producing positive outcomes. The results of this review provide directions for related interventions and future research.