© 2018, The Japanese Society of Hypertension. The aim was to investigate the risk of developing type 2 diabetes according to blood pressure (BP) levels and presence or absence of hypertensive treatment. This 5-year cohort study comprised 3508 Japanese adults aged 30–74 years without diabetes who had undergone a medical checkup including a 75-g oral glucose tolerance test (OGTT) between April 2008 and March 2009 at Saku Central Hospital. Participants receiving antihypertensive treatment were categorized into controlled hypertension (< 140/90 mmHg) or uncontrolled hypertension (≥ 140/90 mmHg) groups. Participants not receiving antihypertensive treatment were categorized: optimal BP (< 120/80 mmHg), normal BP (120–129/80–84 mmHg), high-normal BP (130–139/85–89 mmHg), grade I hypertension (140–159/90–99 mmHg), and grade II/III hypertension (≥ 160/100 mmHg). Hazard ratios and 95% confidence intervals for the incidence of type 2 diabetes as defined by the 75-g OGTT were estimated using multivariable-adjusted Cox proportional hazard models in reference to optimal BP. During an average of 4.4 years of follow-up, 295 participants developed type 2 diabetes. Those with high-normal BP, grade I hypertension, grade II/III hypertension, and uncontrolled hypertension were at significantly higher risk for developing type 2 diabetes, with hazard ratios (95% confidence intervals) of 1.53 (1.03–2.29), 1.53 (1.02–2.32), 2.19 (1.01–4.77), and 1.81 (1.10–2.99), respectively. In conclusion, compared with those with optimal BP, individuals with BP ≥ 130/85 mmHg not receiving antihypertensive treatment and uncontrolled hypertensives with BP ≥ 140/90 mmHg receiving antihypertensive treatment were at a significantly higher risk for developing type 2 diabetes.