Assessing students' multicultural self-efficacy was a challenging duty for high school counselors, particularly in conjunction with the understanding function of school counseling. The main objective of this development research was to design and validate a scale for measuring the multicultural self-efficacy of high school students. The 851 public high school students from seven regions of Indonesia participated as respondents. After reviewing pertaining literature, the study set 40 preliminary items about cultural insight, communication, cultural value, cultural awareness, and flexibility. A school counselor educator and a high school counselor experienced in the field examined its construct validity and suggested improving item clarity. The revised scale had sequential testing to determine its practicality, validity, and reliability values, namely the practicality test, the pilot test, and the main test. The results showed that 36 of 40 items met the validity and reliability criteria. In addition, most respondents were in significant agreement about the scale practicality. This evidence indicated that this assessment instrument was suitable for measuring the multicultural self-efficacy of high school students.