Despite the need for more valid and reliable measures of teacher assessment competencies, few studies have explored these competencies in student assessment. This study aims to empirically investigate the assessment competencies of Korean teachers by utilizing portfolio assessments rated by an expert group, comparing them with the results of teachers’ self-assessments based on self-reported questionnaires. For the analysis, teacher portfolios on student assessment, reflecting teachers’ authentic assessment activities, were collected from 39 secondary school teachers over one semester. The analysis revealed that self-assessment results for all five sub-constructs of teacher competency in student assessment were higher than the results from portfolio assessments, indicating that teachers perceived their own assessment competencies more positively. Additionally, the evaluation using teacher portfolios for student assessment is shown to be advantageous in providing opportunities to understand various assessment theories, sharing assessment methods among teachers, and reflecting on teachers’ overall student assessment activities. Based on these findings, this study explores the applicability of teacher portfolios, focusing on student assessment, for fostering teachers’ assessment literacy.