Aim: The aim of the work was to study the circulating microRNA-133a levels in blood plasma of patients with arterial hypertension (AH), hypertensive heart disease (HHD), and left ventricular (LV) diastolic dysfunction (DD). Materials and Methods: A total of 48 patients with grade 2-3 AH and HHD at the age of 52.23 ± 7.26 (23 patients had LV DD [main group] and 25 patients had normal LV diastolic function [comparison group]) and 21 practically healthy individuals of comparable gender and age were examined. Diagnosis of AH and HHD was carried out according to the 2018 ESC/ESH recommendations. LV DD was determined according to the 2016 ASE/EACVI recommendations. Plasma microRNA-133a level was obtained by polymerase chain reaction using the CFX96 Touch System (BioRad), ≪TaqMan microRNA Assay≫ and ≪TaqMan® Universal PCR Master Mix≫ reagent kits (Thermo Fisher Scientific, USA). Results: We have found that in patients from the main and comparison groups plasma microRNA-133a levels were significantly lower than in practically healthy individuals (0.094 [0.067, 0.147]) and (0.182 [0.102, 0.301]) vs. (0.382 [0.198,0.474]), p = 0.002 and p = 0.04, respectively. In all this among patients with AH, HHD, and LV DD, plasma microRNA-133a levels were significantly lower than in patients with AH, HHD, and normal diastolic function (p = 0.03). In the main and comparison groups there was a statistically significant negative relationship between plasma microRNA-133a level and left ventricular mass index (LVMI) (R = -0.40, p = 0.003 and R = -0.35, p = 0.04, respectively). Conclusions: The findings suggest the significant role of decreased microRNA-133a levels in blood plasma of patients with AH in the pathogenesis and development of both HHD and LV DD.