Background: For the past decade, science educators have been exploring the use of Socio-scientific Issues (SSI) as contexts for science teaching and learning, and research indicates that doing so can support significant learning gains. However, research related to how teachers take up the practice of SSI-based instruction is far more limited, due in part to a lack of tools for use in this kind of research. Purpose: The focus of this research is development and testing of a new classroom observation protocol specifically designed for SSI-based instructional contexts. Design and methods: Development of this SSI-Observation Protocol (SSI-OP) took place in four distinct phases: review of existing protocols and SSI-based instruction frameworks, writing and revision of protocol items, initial testing of the draft protocol, and soliciting feedback from SSI experts. Sample: Following the four stages of SSI-OP development, we progressed to a series of field tests. The field tests were conducted with three different samples. The first sample was an experienced (10+years) high school biology teacher and one of her honors biology classes. The second sample consisted of seven Turkish Pre-service Science Teachers (PST) participating in a science methods course. The third sample included two Thai PST from a field experience course embedded within a teacher education program. Results: The final version of the protocol addressed five dimensions of SSI-based instructional activities: focus of instruction, teaching moves, role of teacher, role of students, and classroom environment. Conclusions: The SSI-OP could be used in a variety of ways for research including documentation of current practices, impacts of professional development and/or curricula on teaching practices, and changes in teaching over time. We offer the SSI-OP as a new tool with the potential to contribute to science teacher education and research that may advance the teaching and learning of science through SSI.