Learning by teaching is a technique where students teach an actual class to facilitate active learning. This paper reports ongoing research involving undergraduate students at an established university in Indonesia. The objective of this quantitative study was to find out the effect of this teaching technique in improving students' 21st century skills. The sample was selected using a cluster sampling technique, where 25 students who were English as a Foreign Language pre-service teachers were assigned to an experimental group, and the other 23 students were treated as a control group. The students in the experimental group were asked to conduct a workshop in groups to students from the same department at a different university. Each group selected one of the topics in their course syllabus. A pre-test measuring the students' soft skills was conducted before the course started and after it finished. The results show that soft skills, which include accountability, communication, innovation, problem-solving, teamwork, professional working, and networking skills, improved significantly among students in the experimental group, but the improvement was not evident in the control group. Therefore, learning by teaching should be embraced as an alternative teaching technique when soft-skill development becomes a target of instruction.