Argumentation is a complex skill essential for English as a Foreign Language (EFL) students to effectively use their English language and reasoning abilities in writing and speaking. Constructing arguments without proper collaborative scaffolding and technological support can be cognitively demanding. Generative artificial intelligence (Gen AI) in learning shows potential to meet students' individual needs. In this quasi-experimental study, we proposed using ChatGPT, a large language model, to guide students through three stages of collaboration script: preparation, interaction, and reflection stages. It ensures active participation and contribute to the collaborative task, while facilitating the knowledge construction in argument mapping to organize and represent the structure of arguments such as clarifying claim. A total of 67 freshmen university students participated, with 34 of them using ChatGPT-supported collaborative argumentation (ChatGPT-CA) and 33 using conventional-based collaborative argumentation (C-CA) which relied on a conventional pedagogical method without Gen AI support. The findings showed that the ChatGPT-CA approach significantly enhanced EFL students' argumentative speaking performance, critical thinking awareness, and collaboration tendency compared to the C-CA approach with large effect sizes (?2 = 0.33; 0.29; 0.33). Furthermore, using ChatGPT to learn argumentation improved the quality of arguments for students at different English proficiency levels. However, limitations such as the influence of prompt quality on ChatGPT's effectiveness were noted. Therefore, ensuring high-quality prompts is crucial for educators. This study provides valuable insights for developing EFL students' argumentation skills based on their proficiency levels and highlights the potential of integrating ChatGPT with proven pedagogical strategies to enhance argumentation skills in EFL contexts.