Students' science identity has been identified as an important construct that positively influences their learning engagement, persistence, and career intentions in science. This paper reported the process of developing and validating an instrument that can be used to assess high school students' science identity. The development of this instrument was based upon four components in the conceptual framework of science identity: performance, competence, recognition, and interest. After the questionnaire was developed, a pilot study was undertaken for 197 high school students. Then the second administration collected data from 215 students for further validation. Both samples included pupils across grades 8, 10, and 11. Results of data analysis indicated that this instrument has strong construct validity and reliability when used with junior and senior high school students. This instrument could be practically useful as a tool for gathering information that can guide teachers in improving their teaching practices and help to evaluate the effectiveness of science educational interventions.