<![CDATA[21st century mathematics learning demands that students be able to think critically and reflect on their thinking process. Formative assessment techniques, such as self-assessment, offer an approach that facilitates metacognitive awareness, but their implementation in the Junior High School (SMP) context has not been widely studied. This study aims to explore the application of self-assessment techniques in mathematics learning and their contribution to students' critical thinking skills. The study used a qualitative case study method on 22 students from a well-accredited private junior high school in Depok City. Data was collected through in-depth interviews, classroom observations, and self-assessment sheets, and then analyzed thematically. The results show that self-assessment encourages students to reflect on problem-solving strategies, recognize mistakes, and improve critical thinking skills. Although students initially showed confusion, teacher support and rubric structure proved to be important in forming reflective habits. This technique also opens up space for internal dialogue and awareness of the mathematical thinking process. The contribution of this research lies in the understanding of the context of implementing self-assessment at the junior high school level and its potential in fostering critical thinking skills on an ongoing basis. Pembelajaran matematika abad 21 menuntut siswa mampu berpikir kritis dan merefleksikan proses berpikirnya. Teknik asesmen formatif, seperti self-assessment, menawarkan pendekatan yang memfasilitasi kesadaran metakognitif, namun implementasinya dalam konteks Sekolah Menengah Pertama (SMP) belum banyak ditelaah. Penelitian ini bertujuan mengeksplorasi penerapan teknik self-assessment dalam pembelajaran matematika dan kontribusinya terhadap kemampuan berpikir kritis siswa. Penelitian menggunakan metode kualitatif studi kasus pada 22 siswa SMP Swasta terakreditasi baik sekali di Kota Depok. Data dikumpulkan melalui wawancara mendalam, observasi kelas, dan lembar self-assessment, lalu dianalisis secara tematik. Hasil menunjukkan bahwa self-assessment mendorong siswa merefleksikan strategi penyelesaian soal, mengenali kesalahan, dan meningkatkan kemampuan berpikir kritis. Meski awalnya siswa menunjukkan kebingungan, dukungan guru dan struktur rubrik terbukti penting dalam membentuk kebiasaan reflektif. Teknik ini juga membuka ruang dialog internal dan kesadaran proses berpikir matematis. Kontribusi penelitian ini terletak pada pemahaman konteks implementasi self-assessment di jenjang SMP dan potensinya dalam menumbuhkan kemampuan berpikir kritis secara berkelanjutan]]>
