Due to the rapid change and development in science and technology, it has come to the fore to educate individuals with 21st-century skills. Scientific argumentation plays important role in developing 21st-century skills. Several studies were conducted on argumentation in science education and laboratory, however, the limited study examined argumentation in the physics laboratory. This study aimed to investigate whether online argumentation in open-ended physics experiments affects the academic achievement and argumentation abilities of preservice science teachers. Quantitative and qualitative research methods were used in this study. In the quantitative parts of the study the quasi experimental design was used. Qualitative data were collected through online argumentation to determine the change in argumentation abilities of students. The participants were consisted of 55 preservice science teachers. The experimental group students participated in online discussion in open-ended experiments and the control group students performed confirmatory experiments. It was found that online argumentation in open-ended experiments was more effective in improving the academic achievement of the students compared to confirmatory experiments. The results also indicated that argumentation abilities of the experimental group students improved throughout the intervention.