Left ventricular (LV) hypertrophy (LVH) is an independent cardiovascular risk factor for heart failure (HF) patients. The renin–angiotensin system plays a key role in LVH, and since olmesartan increases plasma angiotensin-(1–7) through an increase in angiotensin-converting enzyme-related carboxypeptidase (ACE2) expression, it was hypothesized to reduce LVH, unlike other angiotensin II receptor blockers (ARBs). The objective of this study was therefore to investigate the effects of a changeover from other ARBs to olmesartan on LVH in HF patients. Participants enrolled in this prospective trial were 64 outpatients with stable HF who had received ARBs other than olmesartan for more than 1 year (age: 59 ± 13 years). Transthoracic echocardiography and laboratory tests were performed before and 6 months after administration of olmesartan. Other drugs were not changed during follow-up. The primary end point was defined as a change in LV mass index (LVMI) from baseline up to 6 months after administration of olmesartan. No significant changes were observed in blood pressures and heart rate after administration of olmesartan. LVMI showed a significant decrease from 119 ± 38 to 110 ± 24 g/m² (p = 0.007) 6 months after administration of olmesartan, and further decreased from 110 ± 24 to 103 ± 35 g/m² (p = 0.0003) after 12 months. Moreover, this reduction tended to be more prominent in patients with LVH. In conclusions, LVH in HF patients was reduced by the changeover to olmesartan. This finding may well have clinical implications for better management of HF patients.