The scientific competencies advocated by the Programme for International Student Assessment (PISA) focus on the abilities needed in students' adult lives. This study investigated how such scientific competencies could be improved by using online argumentation. One hundred and thirty-eight 8th grade high school students took part in the study, with 69 in the experimental group and 69 in the control group. A quasi-experimental design was adopted and qualitative and quantitative analyses were used. An online argumentation system served as an aid for argumentation instruction and activities among experimental group students during the experiment. The results showed that using online argumentation could improve the students' scores for the PISA scientific competencies. The experimental group students outperformed their counterparts in terms of overall mean scores for the scientific competencies. On the one hand, the individual competencies of "using scientific evidence" and "identifying scientific issues" of the experimental group were higher than those of the control group. On the other hand, the experimental group students did not outperform their counterparts in terms of competency in "explaining phenomena scientifically". Using an online environment to complement argumentation instruction and organizing argumentation activities focused on related topics may be a potential direction to consider for improving students' PISA scientific competencies.