The aims of this study were to explore the relationship of age at menarche with the incidence of hypertension in postmenopausal women and to examine whether the degree of obesity, as measured by body mass index (BMI), mediates the relationship. A total of 15361 postmenopausal women from the Henan Rural Cohort Study were included in this study. Subjects provided information on the age of menarche. Logistic regression analysis was used to estimate the odds ratio (OR) and 95% confidence interval (CI). Mediation analysis was used to examine whether the association between age at menarche and hypertension was mediated by BMI, and calculate the proportion to which BMI contributed to this association. The means of the participant's age at menarche and age at recruitment were 16.1 and 61.5 years, respectively. Each 1-year increment in age at menarche was associated with a 3.0% decrease in the risk of hypertension. Compared with the middle quintile (16 years), those who were in the late menarche quintile (≥18 years) had a lower risk of hypertension (OR, 0.876; 95% CI, 0.788-0.975; P = 0.015) after adjusting for age at recruitment, menopausal age, parity, and other potential confounders. The OR (95% CI) of a significant indirect effect of BMI on the age at menarche-hypertension association was 0.977 (0.973-0.981; P = 0.0020). Women with a history of late menarche had a lower risk of hypertension. BMI mediated the effect of age at menarche on hypertension.