The population of older adults is expected to increase considerably in the coming decades. This increase poses a major public health challenge, as aging is associated with a reduction in physical and cognitive functioning. The decline impedes older adults to live independently, and to maintain their quality-of-life. As a result, the demand for health care will rise dramatically. Emerging evidence shows that sedentary behavior contributes to adverse health outcomes. As such, decreasing sedentary behavior might be a promising strategy to foster healthy aging. Yet, few interventions have targeted older adults’ sedentary behavior. Moreover, existing interventions 1) generally do not take into account the habitual nature of sedentary behavior, 2) are often limited in their ability to be broadly implemented at low cost, and 3) are not dynamically tailored to the individuals’ daily needs. Consequently, the overall objective of the current research project is the development, optimization and evaluation of theory-based mHealth intervention aimed at the reduction of sedentary behavior in community-dwelling older adults. The intervention will be grounded in a dual-process framework, and will delivered via an online application on participants’ smartphone. Short- and long-term intervention effects on sedentary behavior and geriatric-related health outcomes will be evaluated, as well as user engagement.