Penelitian ini dilatarbelakangi oleh permasalahan kurangnya kemampuan self-control dan perilaku asertif siswa dalam lingkungan sekolah. Padahal, self-control dan perilaku asertif merupakan keterampilan penting yang perlu dimiliki seorang anak sejak dini karena memberikan manfaat positif dalam hubungan interpersonal serta pencapaian akademik yang baik. Salah satu cara untuk mengembangkan self-control dan perilaku asertif adalah melalui permainan kooperatif. Sehingga, penelitian ini bertujuan untuk mengembangkan self-control dan perilaku asertif pada siswa melalui permainan kooperatif. Penelitian menggunakan metode Penelitian Tindakan Kelas dengan tiga siklus pada siswa kelas I SD XYZ Jakarta dengan jumlah 33 siswa. Data dikumpulkan melalui telaah dokumen, observasi, wawancara, rubrik, catatan lapangan, dan dokumentasi. Analisis data berupa deskriptif kualitatif melalui langkah-langkah: reduksi data, penyajian data, dan interpretasi data. Hasil penelitian menunjukkan bahwa permainan kooperatif mampu mengembangkan self-control dan perilaku asertif siswa kelas I SD XYZ Jakarta. / This research was conducted due to the lack of students’ self-control and assertiveness at school. However, these two skills are important for children to have from an early age since they provide positive benefits in interpersonal relationships and support good academic achievements. One way to develop self-control and assertiveness is through cooperative play. Thus, this study aims to develop students' self-control and students' assertiveness through cooperative play. In the study, the Classroom Action Research method was used with three cycles for the Grade I students of the XYZ Elementary School with a total of 33 students. The data were collected through document reviews, observations, interviews, rubrics, field notes, and documentations. The data were analyzed in the forms of the qualitative descriptive approach through some steps: data reduction, data presentation, and data interpretation. The results of the research have shown that the implementations of cooperative play may promote the students’ self-control and assertiveness for the Grade I students of the XYZ Elementary School Jakarta.