Ultrasound is an important diagnostic tool in patients with abdominal pain and after injury. However, it is highly dependent on the skills and training of the examiner. Thus, ultrasound competencies should be acquired early during medical education. The instructional approach affects the retention and performance of skills. A promising approach is "mastery learning." The aim of the study was to evaluate the effectiveness of "mastery learning" compared with the "see one, do one" approach by performing a focused assessment of sonography for trauma (FAST) in undergraduate medical students based using an academic assessment tool (Objective Structured Clinical Examination [OSCE]). In a prospective controlled trial, 146 participants were randomly allocated to two groups (see one, do one and mastery learning) and trained in a 90-min module. In the see one, do one group, the trainer demonstrated the complete FAST routine, and then the students trained each other on it under supervision and received direct oral feedback from the tutors. In the mastery learning group, each student received a routing slip. The routing slip contained five levels of competence for the FAST routine, each of which had to be achieved (e.g., choosing the correct probe) and verified by the trainer before working toward the next competency level. The acquired competencies were assessed after training using the OSCE, which is a standardized practical exam using checklists. The mastery learning group attained 40.69 ± 5.6 points on average (of a maximum of 46 points), and the see one, do one group, 33.85 ± 7.7 points (p < 0.001). Mastery learning is an effective teaching method for undergraduate medical students performing FAST and is superior to the see one, do one approach, as assessed with the OSCE.