Aim <p>To investigate, for the first time, the association between undiagnosed diabetes—using a single-sample confirmatory definition—and prevalent hypertension and cardiovascular diseases (CVD) in an Iranian national population. A few studies on this topic have been limited to Western populations.</p> Methods <p>The study included 16328 adults aged ≥ 30 years. Diabetes phenotypes were classified as: 1-no diabetes: fasting plasma glucose (FPG) &lt; 7 mmol/L (126&#xa0;mg/dL) and glycated hemoglobin (HbA1c) &lt; 6.5% (48 mmol/mol), as reference; 2-unconfirmed undiagnosed diabetes: having elevated levels of either FPG or HbA1c; 3-confirmed undiagnosed diabetes: having elevated levels of both tests; 4-known diabetes: self-reported history of glucose-lowering medications. Hypertension was defined using American Heart Association criteria. Self-reported history of CVD is defined as prevalent CVD. Multivariable logistic regression models were applied to estimate the odds ratio (OR) and 95% confidence interval (CI) of the association between diabetes phenotypes with hypertension and CVD.</p> Results <p>Prevalence of unconfirmed undiagnosed-, confirmed undiagnosed-, and known -diabetes was calculated as 2.70, 3.10, and 5.64%, respectively. Unconfirmed undiagnosed-, confirmed undiagnosed-, and known -diabetes were associated with hypertension with corresponding OR were, 1.30, 1.37, and 1.62; the related values for CVD were 1.80, 1.61, and 2.38; and for the coexistence of CVD and hypertension were 1.86, 1.79, and 2.80, respectively (all P _values &lt; 0.05). Furthermore, isolated HbA1c elevation was significantly associated with prevalent CVD [2.04 (1.20–3.45)], and coexistence of hypertension and CVD [1.89 (1.00–3.55)].</p> Conclusions <p>Both unconfirmed- and confirmed- undiagnosed -diabetes were significantly associated with hypertension and CVD, the issue that was mainly attributable to high HbA1c.</p> Graphical abstract <p></p>