Teachers lack effective curriculum-based instruments to assess their students' scientific competence that would provide information for modifying their inquiry instruction. The main purpose of this study was to develop and validate a Curriculum-Based Scientific Competence (CBSC) test to assess students' scientific competence in a 1-semester Grade 9 curriculum. The initial 51-item CBSC test was administered to 318 Grade 10 students from 4 schools. The Rasch partial credit model was used to analyse the quality of the CBSC test. The final 44-item CBSC test demonstrated satisfactory item fit, threshold, and test reliability. Further, a 23-item PISA test was used to measure criterion validity. Results indicated that the CBSC test was valid for assessing students' scientific competencies and can distinguish the effect of inquiry instruction from that of traditional instruction. A CBSC test would provide teachers the summative assessment of scientific competency for examining the effect of inquiry-based teaching. The model of development and validation of the CBSC test through the cooperation of teachers and researchers for each semester of a science programme is recommended.