Background: Informal workplace learning (WPL) has no concrete learning objective and takes place without a responsible supervisor, which makes it difficult to assess its learning outcomes. Formal learning situations, as they are known from universities or schools, do not exist in this context and make a conventional assessment of learning goals and achievements impossible. Informal learning in the workplace is of central importance, and the assessment of informal learning outcomes in medical education is an under-researched area. The aim of our study was to adapt and validate an informal WPL questionnaire (originally developed for social workers) to assess learning outcomes due to informal WPL in residency training. Methods: A total of 528 residents (n = 339 female; age: M = 29.79; SD = 3.37 years) completed an adapted questionnaire on informal WPL outcomes and the Freiburg Questionnaire to Assess Competencies in Medicine (i.e. medical knowledge, communication, and scholarship). Exploratory factor analysis was used to determine the underlying factor structure. The reliability of the factors was tested using McDonald’s omega, and the correlation between the factors and the three subscales of the Freiburg questionnaire was tested using Spearman’s rho correlation coefficient. To investigate construct validity, a structural equation model was calculated to examine the relationships between medical competencies and informal learning outcomes. Results: The exploratory factor analysis yielded a four-factor solution that best fit the data. The scores of all four factors (GLO-CD: generic learning outcomes—competence development, GLO-R: generic learning outcomes—reflection, JSLO: job-specific learning outcomes, and OLLO: organisational learning outcomes) showed good internal consistency (Ω ≥.69). The structural equation model showed that "medical expertise" had an impact on all four factors of informal learning at work. “Scholarship” seemed to predict GLO-CD and GLO-R. Conclusions: Our four-factor model reveals meaningful determinants of informal WPL in relation to residency training. The instrument is therefore the first promising attempt to assess informal WPL in the broader context of medical education during residency, thus supporting its construct validity.