Background: The competency-based approach to the assessment of nursing practice has been adopted as a key policy in the developed world. The continual self-assessment of competence gives nurses the opportunity to reflect on their competencies and has a significant impact on the quality of nursing practice and patient safety. The study was designed to describe the process of cross-cultural adaptation and to assess the psychometric properties of the Slovenian version of a short form of the Nurse Professional Competence scale (NPC-SF) and to evaluate the efficacy of this instrument in a sample of registered nurses. Methods: A cross-sectional and validation study was conducted in 425 registered nurses to test the psychometric properties of the Slovenian version of a short form of the scale and to evaluate nurses’ professional competence. A multilevel approach was used: Translation, back-translation, language validity, face and content validity, construct validity, and reliability of the Slovenian version of the scale were analysed respectively. Participants completed an online survey, with the data being collected between April and July 2020. Results: Factor analysis showed that the Slovenian version of the scale could be used in four dimensions explained with 65 % of the variance. Cronbach’s α was 0.972. The four-factor model fit the data (RMSEA = 0.083, CFI = 0.731). Self-reported competence was high and rated higher by nurses employed at the tertiary level of healthcare, followed by nurses employed at the secondary and primary, and from social care institutions. Nurses with more years of experience assessed their competence higher. Conclusions: The NPC-SF helps understand and identify nurses’ self-reported core competencies in clinical settings, thereby providing an important predictor of the professional development of nursing. The Slovenian version of the scale demonstrated acceptable psychometric properties and may be used in research and clinical practice to evaluate nurses’ professional competence.