The increasing older adult population and the importance of their mental health have highlighted the need to examine factors associated with the psychological well-being of this population. As a widely used technology, smartphones may be associated with older adults’ psychological status by providing opportunities for communication, access to information, services, and leisure activities. However, existing evidence regarding the psychological consequences of smartphone use is not consistent, suggesting the importance of considering both the amount and domains of use. Therefore, the present study aims to determine the direct and indirect relationships between smartphone use patterns and symptoms of depression and anxiety among older adults in Tehran, considering the mediating role of sense of belonging. This study is quantitative and correlational in design. The study population will consist of older adults aged 60 years and above residing in Tehran in 1405, and 312 participants will be selected through convenience sampling. Data will be collected using a demographic questionnaire, a researcher-developed smartphone use pattern checklist, the Geriatric Depression Scale, the Geriatric Anxiety Inventory, and a sense of belonging instrument. Data will be analyzed using descriptive statistics and structural equation modeling in SPSS and AMOS, and bootstrapping will be used to examine indirect effects. The findings may contribute to a more precise understanding of the relationship between smartphone use patterns, sense of belonging, and the mental health of older adults.