Background : Assessing competence of advanced undergraduate medical students based on performance in the clinical context is the ultimate, yet challenging goal for medical educators to provide constructive alignment between undergraduate medical training and professional work of physicians. Therefore, we designed and validated a performance-based 360-degree assessment for competences of advanced undergraduate medical students. Methods: This study was conducted in three steps: 1) Ten facets of competence considered to be most important for beginning residents were determined by a ranking study with 102 internists and 100 surgeons. 2) Based on these facets of competence we developed a 360-degree assessment simulating a first day of residency. Advanced undergraduate medical students (year 5 and 6) participated in the physician's role. Additionally knowledge was assessed by a multiple-choice test. The assessment was performed twice (t₁ and t₂) and included three phases: a consultation hour, a patient management phase, and a patient handover. Sixty-seven (t₁) and eighty-nine (t₂) undergraduate medical students participated. 3) The participants completed the Group Assessment of Performance (GAP)-test for flight school applicants to assess medical students"facets of competence in a non-medical context for validation purposes. We aimed to provide a validity argument for our newly designed assessment based on Messick's six aspects of validation: (1) content validity, (2) substantive/cognitive validity, (3) structural validity, (4) generalizability, (5) external validity, and (6) consequential validity. Results: Our assessment proved to be well operationalised to enable undergraduate medical students to show their competences in performance on the higher levels of Bloom's taxonomy. Its generalisability was underscored by its authenticity in respect of workplace reality and its underlying facets of competence relevant for beginning residents. The moderate concordance with facets of competence of the validated GAP-test provides arguments of convergent validity for our assessment. Since five aspects of Messick's validation approach could be defended, our competence-based 360-degree assessment format shows good arguments for its validity. Conclusion: According to these validation arguments, our assessment instrument seems to be a good option to assess competence in advanced undergraduate medical students in a summative or formative way. Developments towards assessment of postgraduate medical trainees should be explored.