Introduction: Ultrasound diagnostics is an important examination method in everyday clinical practice, but student education is often inadequate for acquiring sufficient basic skills. Individual universities have therefore started integrating (extra)curricular training concepts into medical education. This study aimed to evaluate sustainable skills development through participation in peer-assisted ultrasound courses. Methods: From 2017, students in the clinical part of medical school could opt for extracurricular peer-assisted ultrasound courses. Depending on the format (10-week course/2-day compact course) these comprised 20 teaching units focusing on abdominal and emergency ultrasonography. Students attending compulsory workshops at the start of their practical year were enrolled in this study, allowing for a comparison between the study group (attended ultrasound course) and the control group (did not attend ultrasound course). Competency from two out of four practical exams (subjects: “aorta”, “gallbladder”, “kidney” and “lung”) was measured, and a theory test on the same subject areas (“pathology recognition”) was administered. Additional questions concerned biographical data, subjective competency assessment (7-point Likert scale), and “attitude to ultrasound training in the curriculum”. Results: Analysis included 302 participants in total. Ultrasound courses had been attended on average 2.5 years earlier (10-week course) and 12 months earlier (2-day compact course), respectively. The study group (n = 141) achieved significantly better results than the control group (n = 161) in the long-term follow-up. This applies both to practical exams (p < 0.01) and theory tests (p < 0.01). After course attendance, participants reported a significantly higher subjective assessment of theoretical (p < 0.01) and practical (p < 0.01) ultrasound skills. Conclusions: Peer-assisted ultrasound courses can sustainably increase both theoretical and practical competency of medical students. This highlights the potential and need for standardised implementation of ultrasound courses in the medical education curriculum.