Background: Advancements in medical care have increased the average life span in many countries, resulting in a generally longer postretirement life span. However, retirees may find it difficult to adapt to retirement. Therefore, encouraging retirees to engage with society is important. Purpose: In this study, a senior social participation mobile software application (SSP-App) was developed to stimulate social participation among seniors with the goal of improving their social participation intentions and behaviors. Methods: After developing the SSP-App based on user experiences, a quasi-experimental study was conducted. Participants were recruited from the Keelung Ren'ai Community Center. Next, Random Allocation Software Version 1.0.0 software was used to randomly allocate the participants into experimental and control groups. The 54 participants in the experimental group took part in an SSP-App program, whereas the 53 participants in the control group did not participate in any experimental treatment program. Measurements were conducted at Week 4 (T1) and Week 12 (T2) to evaluate the effects. Data were collected using a demographic datasheet, Geriatric Depression Scale-Short Form, Emotional and Social Support Scale, Social Participation Intention Scale, and Social Participation Behavior Scale. The generalized estimating equations method was used to determine intervention effectiveness. Results: The SSP-App has six main functions, including an activity partner message board, an activity search function that provides information about different activities, a “Seniors Learning Kiosk” that provides useful information, transportation information, an activity planning and reminder system, and a “First- Aid Station.” Most participants in the SSP-App precursor test expressed approval. At T1, effects were observed in social participation intention only. However, at T2, effects were observed in both social participation intention and social participation behavior. Conclusions/Implications for Practice: The SSP-App developed in this study uses information and communication technology and multiple strategies covering information provision, social support, education, and reminders. Social participation obstacles must be overcome to effectively provide seniors with social participation opportunities and improve their social participation.