Critical thinking skills are needed to face the challenges of living in the 21st century. Education must be able to foster students' critical thinking and enable them to think independently. This study aims to investigate the development of students' critical thinking skills during the learning process using an e-module based on problem-based learning combined with Socratic dialogue. This qualitative research study measured students' critical thinking skills during the learning process using Socratic dialogue questions on a learning activity sheet. The students' answers on their learning activity sheets were analyzed qualitatively using a coding process. The results show that the application of an e-module based on problem-based learning combined with Socratic dialogue can improve students' critical thinking skills. During the learning process, all aspects of critical thinking skills (i.e., interpretation, analysis, evaluation, inference, explanation, and self-regulation) improved over the duration of the study.