Hypertension is an important risk factor for cardiovascular disease, the leading cause of mortality in the world. Although previous studies have focused on individual-level behavioral risk factors associated with hypertension, there has been little research on how interacting with others, that is social participation, affects hypertension. To address this research gap, this study examined the association between social participation and hypertension in Japan, a country with a high prevalence of hypertension possibly linked to rapid population aging. Data were used from 4582 participants aged more than 65 years who participated in the Japan Gerontological Evaluation Survey (JAGES) with blood pressure data collected during a health check-up. The frequency of participation in vertical organizations (characterized by hierarchical relationships) and horizontal organizations (characterized by non-hierarchical, egalitarian relationships) was measured by a questionnaire. In a Poisson regression analysis, participation in vertical organizations was not associated with hypertension, whereas participation in horizontal organizations at least once a month was inversely associated with hypertension (prevalence ratio: 0.941). This association remained significant after adjusting for social support variables, although further adjustment for health behaviors attenuated the association. As the frequency of going out and average time spent walking were both associated with hypertension, physical activity may be a possible pathway that connects social participation and hypertension. The results of this study suggest that expanding social participation programs, especially those involving horizontal organizations, may be one way to promote better health among older people in Japan.