This quasi-experimental study examines the effects of English debate activities on Korean university students’ written and oral argumentation skills. A pretest-posttest control group design was utilized, comprising an experimental group (n = 46) that engaged in weekly English debate sessions and a control group (n = 33) that received alternative English instruction. Data were collected through written and oral argumentative opinion tasks administered before and after the semester. The participants' argument structures were analyzed using a modified version of Toulmin's model of argumentation. ANCOVA was conducted to compare the posttest performances of the two groups, incorporating pretest scores as covariates. The findings indicated that the experimental group significantly outperformed the control group in overall argumentation skills. Specifically, the experimental group showed notable improvements in their ability to use warrants, present counterarguments, and incorporate qualifiers, reflecting a deeper understanding and more effective construction of arguments. These enhancements were consistently observed across both written and oral tasks, demonstrating the transferability of skills. The study underscores the effectiveness of debate as a pedagogical method for enhancing critical thinking and argumentation in the context of English as a Foreign Language (EFL) and discusses the implications of integrating debate activities into EFL curricula.