Open innovation is a paradigm proposing that companies can benefit from using external pathways and knowledge and that firms should cross their boundaries in the product development and innovation management. Since several firms have gained success from practicing open innovation, the paradigm has received high attention from the academia and businesses.  However, in spite of the service sector’s high economic contributions, open innovation in the sector is still under-researched. This study focuses on open innovation stages in the service sector and identifies the important processes and barriers in each stage. The results include four stages of open innovation implementation and insights on processes including process selection, partner selection, goal setting and influential factor control. The information used in this study came from the interviews with service companies in Thailand from three industries, which include financial intermediation, telecommunications and knowledge-intensive industries. The study closes the gap in the academic area as well as provides managerial insights and guidelines for service companies to adopt open innovation more effectively.