[Bahasa]: Penelitian ini bertujuan untuk menganalisis proses pembelajaran siswa introvert pada materi Phytagoras dan hasil perlakuan dengan problem-based learning (PBL) terhadap kemampuan berpikir kritis matematis siswa. Penelitian kuantitatif deskriptif dengan desain Single Subject Research (SSR) A-B digunakan untuk mencapai tujuan penelitian tersebut. Subjek dalam penelitian ini adalah seorang siswa kelas VIII SMP. Data dikumpulkan melalui tes, observasi, dan wawancara.  Selanjutnya, data yang dikumpulkan dianalisis berdasarkan dua kondisi besar selama proses penelitian, yaitu analisis dalam kondisi dan antar kondisi. Analisis dalam kondisi meliputi komponen panjang kondisi (panjang interval), kecenderungan arah, tingkat stabilitas, tingkat perubahan, jejak data, dan rentang. Analisis antar kondisi meliputi jumlah variable yang diubah, perubahan kecenderungan arah dan efeknya, perubahan stabilitas dan efeknya, perubahan level data, serta data yang tumpang tindih (overlap). Hasil penelitian menunjukkan bahwa PBL memiliki peran yang sangat besar selama proses pembelajaran karena mendukung siswa introvert untuk aktif dalam menyelesaikan permasalahan-permasalahan kontekstual yang diberikan, sehingga pada akhir pembelajaran kemampuan berpikir kritis matematis siswa tersebut dapat ditingkatkan. Kata kunci: Berpikir Kritis; Introvert; Single Subject Research; Problem-based Learning [English]: The research aimed to analyze how introvert student learn Phytagoras topic and the result of learning intervention using problem-based learning (PBL) on the student's mathematical critical thinking ability. Descriptive quantitative research with single subject design was used to achieve the aims of the research. The subject was an introvert student of grade VIII. Data was collected through test, observation, and interview. Data analysis was based on two major conditions during the research, i.e. within-conditions and inter-conditions. Within-condition analysis included interval length, direction tendency, stability level, rate of change, data trace, and range. Meanwhile, inter-condition analysis comprised the number of variables changed, changes in direction and effect trends, changes in stability and its effects, changes in data levels, and overlap data. The research showed that PBL has a significant role during the learning process since it supports the subject to be active in solving the contextual problems. In this case, the mathematical critical thinking ability can be developed and improved. Keywords:  Critical Thinking; Introvert; Single Subject Research; Problem-based Learning