We aimed to quantify the impact of inadequate pharmacological therapy on uncontrolled blood pressure (BP) using Japanese real-world data. This retrospective cohort study used databases provided by DeSC Healthcare, Inc (Tokyo, Japan). We identified 27,652 patients with hypertension (age, 60.7 ± 9.1 years; men, 56.4%) who were not receiving antihypertensive treatment at the initial visit (pre-treatment) and were under treatment at the next visit (post-treatment). Patients were classified into the following groups by the number of antihypertensive drug classes and defined daily dose (DDD): one antihypertensive drug class with a low dose (DDD < 1.0), one antihypertensive drug class with a moderate-to-high dose (DDD ≥ 1.0), two antihypertensive drug classes with a low dose (DDD < 2.0), two antihypertensive drug classes with a moderate-to-high dose (DDD ≥ 2.0), and ≥three antihypertensive drug classes. The pre-treatment systolic/diastolic BP was 157.7 ± 15.4/94.2 ± 11.5 mmHg. Overall, 43.0% of patients had uncontrolled BP (post-treatment BP ≥ 140/ ≥ 90 mmHg). High pre-treatment BP was a strong factor for uncontrolled BP. After adjustments for covariates, including the pre-treatment mean BP, the proportion of patients with uncontrolled BP was 2.08 times higher in the one antihypertensive drug class with a low dose group than in the ≥three antihypertensive drug classes group. The preventable fraction due to <three antihypertensive drug classes for uncontrolled BP was 40.6%. Only 9.9% of patients with the pre-treatment BP ≥ 180/ ≥ 110 mmHg were prescribed ≥ three antihypertensive drug classes. High pre-treatment BP and inadequate antihypertensive treatment were major factors contributing to uncontrolled BP. Adequate treatment intensification would resolve approximately 40% of uncontrolled BP cases among Japanese patients treated for hypertension.