KCNQ1 encodes a potassium voltage‐gated channel and represents a susceptibility locus for type 2 diabetes mellitus (T2DM). Here, we explored the association between KCNQ1 polymorphisms and hypertension risk in individuals with T2DM, as well as the role of KCNQ1 in vascular smooth muscle cell contraction in vitro. To investigate the relationship between KCNQ1 and the risk of developing hypertension in patients with T2DM, we divided the T2DM cohort into hypertension (n = 452) and non‐hypertension (n = 541) groups. The Mann–Whitney U test, chi‐square test, and multivariate regression analyses were used to assess the clinical characteristics and genotypic frequencies. In vitro studies utilized the rat aortic smooth muscle A10 cell line. Patients in the hypertension group were significantly older at the time of enrollment and had higher levels of body mass index, waist‐to‐hip ratio, and triglyceride than those in the non‐hypertension group. The KCNQ1 rs3864884 and rs12576239 genetic variants were associated with hypertension in T2DM. KCNQ1 expression was lower in the individuals with the CC versus the CT and TT genotypes. Smooth muscle cell contractility was inhibited by treatment with a KCNQ1 inhibitor. These results suggest that KCNQ1 might be associated with hypertension in individuals with T2DM. The KCNQ1 rs3864884 and rs12576239 genetic variants were associated with hypertension in T2DM. Smooth muscle cell contractility was inhibited by treatment with a KCNQ1 inhibitor. KCNQ1 might be associated with hypertension in individuals with T2DM.