Background: The cultivation and assessment of the professional competence of clinical undergraduates is essential to medical education. This study aimed to construct a scale to evaluate the professional competence of clinical undergraduates as well as its determinants. Methods: The competence scale was developed on the basis of four medical education standards, the literature, and expert interviews. A total of 288 undergraduates from two types of medical colleges in central and southeastern China were selected by a multistage sampling strategy. Factor analysis, correlation analysis, and internal consistency reliability were used to verify the validity and reliability of the scale. Results: A scale consisting of eight factors with 51 items was determined for factor analysis. Cronbach’α coefficients among the eight dimensions were over 0.800, with mean scores of 1.76, 1.38, 1.92, 1.54, 1.77, 1.25, 1.60, and 2.34. Clinical undergraduates with above average academic grades achieved a higher score in essential clinical knowledge (p < 0.05) and better professionalism was reported among females (p < 0.05). Conclusion: The competence scale showed excellent reliability and validity. Respondents in this study showed a moderate level of professional competence. This study could be a reference for medical educators and policy makers in order to improve medical education standards for clinical undergraduates in China and other countries with similar settings.