Ubiquitous mobile social service allows users to interact and communicate with others at any time and place, which is different from the mobile social service in the past. It has become the mobile service most widely used by consumers since various mobile applications (abbreviated as “apps”) were introduced into the market. Recently, the number of elderly users of ubiquitous mobile social service aged over 60 has largely increased. However, the needs and the adoption motivations of the elderly people for this service seemed to have been neglected in the development of mobile devices or services, even in the academic research. For the purpose of understanding what factors make the elderly mobile users willing to adopt ubiquitous mobile social service, this study conducted an empirical research the integrating uses and gratification theory and the media richness theory. Totally, 226 effective questionnaires were obtained, among these questionnaires, there are 193 samples over the age of 60. The analysis results indicated that social, enjoyment and fashion motivations have influences on elderly mobile users' adoption of ubiquitous mobile social service. Furthermore, users' perceived interactive richness and apps self-efficacy also have strong influences on elderly mobile users' adoption of the service. However, the required high expenses make the elderly mobile users keep using their habitual ways to socialize with others, and therefore have a negative influence on elderly mobile users' intention to adopt ubiquitous mobile social service. •Integrating Uses and Gratification Theory with Media Richness Theory.•Analyzing the motivations of elderly users' adoption toward mobile social service.•Except for epistemic, all the motivations have influence on elderly users' adoption.•Perceived interactive richness and self-efficacy are important for elderly users.•Users' inertia of sociality in traditional ways has negative influence on adoption.