Open innovation (OI) is a strategy that firms adopt to innovate by incorporating knowledge from both outside and inside their firms, exploiting their knowledge, and exploring the knowledge of their environment. OI is relevant for small-and medium-sized enterprises (SMEs) because it allows them to adapt and thrive in increasingly turbulent and dynamic competitive environments, generate competitive advantages, and increase their performances. The method adopted for measuring OI is heterogeneous because of the breadth of the concept and application of various OI metrics and practices. This study analyzes the various methods used for measuring OI in the SME context through a systematic review of the empirical literature. To meet this objective, a two-step methodological approach was implemented: first, a systematic literature review, and second, a bibliometric analysis. Finally, 125 empirical articles from 2009 to 2020 were selected from the Web of Science and Scopus databases. The results show that the empirical literature uses a wide variety of methods to measure OI activities. While this adds to the richness of the concept, it makes theory development difficult. Based on the systematic literature review conducted, it is clear that there are different perspectives pertaining to the measurement of OI: (1) external knowledge sources, internal knowledge, and collaboration; (2) technology exploitation and technology exploration; (3) inbound, outbound, and couple; and (4) openness. This study has important implications to researchers and SME managers, will help them develop a better understanding of how OI activities can be measured.