AbstrakLatar belakang penelitian adalah rendahnya kemampuan penalaran dan komunikasi matematis siswa. Penelitian ini bertujuan untuk mengetahui bagaimana kemampuan penalaran dan komunikasi matematis siswa kelas XI MIA (Matematika dan Ilmu Alam) SMAN 1 Koto Salak Kabupaten Dharmasraya. Metode penelitian adalah penelitian deskriptif dengan pendekatan kualitatif. Instrumen yang digunakan berupa tes, wawancara, dan observasi. Subjek penelitian adalah siswa kelas XI MIA SMAN 1 Koto Salak sebanyak 27 orang. Hasil penelitian menunjukkan kriteria kemampuan penalaran dan komunikasi matematis siswa sangat baik sebanyak 3 orang, baik sebanyak 6 orang, cukup sebanyak 7 orang, kurang sebanyak 9 orang, dan sangat kurang sebanyak 2 orang. Jadi, dapat disimpulkan bahwa kemampuan penalaran dan komunikasi matematis siswa kelas XI MIA SMAN 1 Koto Salak dominan berada pada kriteria kurang baik. AbstractThe background of the research is the low mathematical reasoning and communication skills of students. This study aims to assess the mathematical reasoning and communication skills of students of class XI in the Science program of Senior High School No 1 (SMAN 1) in Koto Salak, Dharmasraya Regency. The research method used was a descriptive study with a qualitative approach while the instruments used are in the form of tests, interviews, and observations, with 27 students as the research subjects. The results showed that the criteria of students’mathematical reasoning and communication skills as follows; very good as many as 3 people, good as many as 6 people, fairly good as many as 7 people, bad as many as 9 people, and very bad as many as 2 people. Therefore, it can be concluded that the mathematical reasoning and communication skills of students in class XI in the Science Program of Senior High School No. 1 Koto Salak are in poor criteria.