Designing experiments and applying the process of science are core competencies for many introductory courses and course-based undergraduate research experiences (CUREs). However, experimental design is a complex process that challenges many introductory students. We describe the development of a tool to assess interrelated experimental design (TIED) in an introductory biology lab course. We describe the interrater reliability of the tool, its effectiveness in detecting variability and growth in experimental-design skills, and its adaptability for use in various contexts. The final tool contained five components, each with multiple criteria in the form of a checklist such that a high-quality response—in which students align the different components of their experimental design—satisfies all criteria. The tool showed excellent interrater reliability and captured the full range of introductory-student skill levels, with few students hitting the assessment ceiling or floor. The scoring tool detected growth in student skills from the beginning to the end of the semester, with significant differences between pre- and post-assessment scores for the Total Score and for the Data Collection and Observations component scores. This authentic assessment task and scoring tool provide meaningful feedback to instructors about the strengths, gaps, and growth in introductory students’ experimental-design skills and can be scored reliably by multiple instructors. The TIED can also be adapted to a number of experimental-design prompts and learning objectives, and therefore can be useful for a variety of introductory courses and CUREs.