This Community Service Program (PkM) aimed to enhance the competencies of elementary school teachers in designing and implementing Problem-Based Learning (PBL)-based assessment in Pringsewu Regency, Indonesia. The program was motivated by teachers' limited ability to develop authentic assessments aligned with the characteristics of PBL, particularly in designing project assessment rubrics, observation sheets, and instruments for measuring critical thinking and problem-solving skills. The program involved 35 elementary school teachers and was implemented through several stages, including preparation, training, assessment instrument development workshops, implementation mentoring, and evaluation. A participatory approach was employed through lectures, discussions, simulations, and hands-on practice in developing PBL assessment instruments. The training covered the concepts of PBL assessment, authentic assessment, project rubric development, student activity observation sheets, and assessment instruments for critical thinking and problem-solving skills. The results indicated that 90% of the participants successfully developed PBL-based assessment instruments that met the implementation criteria. Evaluation findings revealed a significant improvement in participants' competencies, with the average score increasing from 62.4 in the pre-test to 86.7 in the post-test, representing a 38.9% improvement. Furthermore, 78% of the participants achieved the "excellent" category, while 17% were categorized as "good." These findings demonstrate that the training program was effective in improving teachers' competencies in developing authentic, contextual, and 21st-century skills-oriented assessment practices.