Background: Over the coming decades, middle-income countries (MICs) are expected to experience substantial population aging. With much of global health research focusing on the economic and mortality burden of aging, the implications of this population growth among older age groups on the future number of individuals in need of healthcare for aging-related diseases often go unappreciated. Using a novel mathematical approach as well as nationally representative blood pressure (BP) data among adults aged 40 years and older from six of the most populous MICs, we estimate - under a range of plausible epidemiological scenarios - the change in the number of individuals in need of hypertension care between 2015 and 2050 that is due to changes in population size, age composition, and disease prevalence. Methods: We used nationally representative household survey data collected between 2012 and 2016 in Brazil, China, India, Indonesia, Mexico, and South Africa to estimate hypertension prevalence for each age group in each country. To reflect unmet need for healthcare, we defined hypertension as systolic BP ≥140 mmHg or diastolic BP ≥90 mmHg regardless of treatment status. Using data on age-specific counts of individuals from the United Nations World Population Prospects, along with a mathematical disease projection equation that we developed, we calculated the change in the number of individuals with hypertension in each country that was due to changes in population size, age composition, and age-specific prevalence under each of four scenarios. These scenarios were that, by 2050, the age-specific hypertension prevalence: i) increases by 10%, ii) remains unchanged, iii) decreases by 10%, and iv) decreases by 25%. Findings: 770,121 adults without a missing BP measurement were included in the analysis. If the current age-specific prevalence schedule of hypertension remains unchanged until 2050, demographic changes alone will increase the number of adults with hypertension by 55% in China, 97% in Brazil, 101% in Indonesia, 105% in South Africa, 108% in India, and 151% in Mexico, corresponding to 319.7 million more individuals across the six countries. If the age-specific prevalence of hypertension is reduced by 25% by 2050 among adults aged ≥40 years, the number of individuals with hypertension will still increase by 145.9 million more individuals, with the relative increase ranging from 16% in China to 88% in Mexico. Interpretation: In MICs, coming demographic changes will vastly overpower potential prevention and health services improvements for hypertension. Large expansions in chronic disease care services will be needed in MICs to meet the inevitable rise of hypertension and other aging-related diseases, such as diabetes and dementia. Funding: Harvard Medical School Center for Global Health Delivery. Declaration of Interest: The authors declare no conflicts of interest.