This study investigated project-based learning approaches for teaching argumentative writing in Indonesian elementary schools. It, therefore, focused on three dimensions of competency: the student's knowledge, ability, and attitude during the learning process. A case study research design with a qualitative approach was adopted to answer the research questions and provide an in-depth explanation of the events occurring within the learning process. Some 32 participants were observed and interviewed in East Java, Province, Indonesia, comprising four teachers and 24 elementary school children, to learn about their experiences when teaching or learning. Three findings were discerned: (i) the project-based learning model is implemented over six stages and focuses on group assignments for writing argumentatively on specific topics; (ii) the students expressed that this learning model was delightful for them, with them being rather enthusiastic about responding to questions, working together, and listening in lessons; and (iii) the students' writing skills improved in terms of the teacher's assessments of assignments. This learning approach for teaching argumentative writing is highly effective at developing aspects of student competency at the primary school level, mainly writing ability. Based on the findings of this study, school principals are recommended to adopt learning activities based on this paradigm, while teachers can use it to enhance control over classroom settings and ensure that students are not distracted from learning. Future research could look into other successful learning models for improving pupils' writing skills.