This paper presents the design and development of a rubric for assessing mathematical modelling for mathematical modelling tasks at the secondary level. A rubric was crafted based on the mathematical modelling competencies synthesised by the researchers identified from four sources. The rubric was fine-tuned following an interview with three practising mathematics teachers, who then used the refined rubric to score students' work on a mathematical modelling task. Post-assessment interviews were conducted with the teachers to check the usefulness of the rubric and the discrepancy of the scores. An interrater reliability analysis, which was carried out during the study, showed that teachers could grade students' work reliably.