Improving science, technology, engineering and mathematics (STEM) curricula and promoting the participation of historically underrepresented groups, including girls in STEM-related majors and careers, have become important policy initiatives in many countries. Although informal learning activities are essential for students, research shows that teachers’ use of innovative pedagogical and assessment approaches in the classroom learning environment has the greatest impact on student learning and choices of STEM-related fields of study and careers. Therefore, there is an urgent need to build teacher capacity in the design and development of instructionally sensitive, authentic assessments with the potential to promote the engagement of elementary school girls in STEM learning. This article is part of a larger design-based research study on building capacity for Canadian elementary school teachers in the design and implementation of authentic STEM assessments to promote girls’ STEM self-efficacy and interest in STEM. Participants included three Grades 5 and 6 teachers and their students. Data sources included in the analysis and results sections of this article comprise teacher-designed assessment tasks, teacher interviews, classroom observations and student self-reflections. Findings indicate that despite their capacity to design assessment tasks with a real-world problem, teachers tended to focus on the solicitation of students’ factual and procedural knowledge, lacked the capacity to promote students’ integration of mathematics and sciences and encountered some implementation challenges. Although there were positive effects of the authentic STEM assessment on girls’ development of a growth mindset, interest in mathematics and investigative skills, teachers should be more intentional in increasing the intellectual engagement of girls in STEM through the creation of authentic assessment tasks that are more gender-responsive and that focus on eliciting higher-order cognitive skills. These findings underscore the importance of providing elementary school teachers with sustained professional development to build their capacity to design and implement high-quality authentic STEM assessments (i.e., high cognitive demand tasks) for girls.