Problem-posing has been recognized for enhancing students' engagement, problem-solving skills, and mathematical knowledge, which leads to higher mathematics achievement. This research is experimental research with a randomized comparison groups pre-test and post-test design, which aims to examine the effect of using online problem-posing to develop students' mathematics achievement during the outbreak of the COVID-19 pandemic in Thailand. Sixty electrical engineering students of Rajamangala University participated in this study. The research instruments included a mathematics problem-posing lesson plan and a mathematics achievement test. The participants were tested for mathematics achievement before and after attending the programs. The result indicated a statistically significant difference in the mathematics post-test mean scores between the experimental and control groups. The experimental group gained higher mathematics achievement than the control group. Therefore, it is recommended that the problem-posing be used in teaching mathematics to the students.