Mathematics anxiety merupakan salah satu faktor yang dapat memengaruhi kemampuan siswa dalam menyelesaikan asesmen matematika. Penelitian mengenai mathematics anxiety dalam konteks asesmen formatif masih terbatas, khususnya yang menganalisis aspek emosional, kognitif, fisiologis, dan perilaku pada materi bangun ruang sisi datar yang menuntut kemampuan visualisasi dan penalaran geometri. Penelitian ini bertujuan menganalisis mathematics anxiety siswa berdasarkan aspek emosional, kognitif, fisiologis, dan perilaku dalam asesmen formatif pada materi bangun ruang sisi datar. Penelitian menggunakan pendekatan kualitatif dengan melibatkan 35 siswa kelas VII SMP Muhammadiyah 5 Surakarta. Data dikumpulkan melalui angket kecemasan matematis, tes asesmen formatif, observasi, dan wawancara. Berdasarkan hasil angket, siswa dikelompokkan ke dalam kategori kecemasan rendah, sedang, dan tinggi, kemudian dipilih masing-masing satu siswa dari setiap kategori menggunakan teknik purposive sampling untuk dianalisis secara mendalam. Data dianalisis melalui reduksi data, penyajian data, dan penarikan kesimpulan, sedangkan keabsahan data diperoleh melalui triangulasi teknik. Hasil penelitian menunjukkan bahwa siswa dengan kecemasan rendah mampu menyelesaikan soal secara sistematis dan percaya diri, siswa dengan kecemasan sedang masih menunjukkan keraguan dalam mengambil keputusan matematis, sedangkan siswa dengan kecemasan tinggi mengalami hambatan pada aspek emosional, kognitif, fisiologis, dan perilaku yang berdampak pada kesulitan memahami masalah, menentukan strategi penyelesaian, dan menyelesaikan perhitungan secara tuntas. Penelitian ini berkontribusi memberikan gambaran komprehensif mengenai mathematics anxiety siswa berdasarkan keempat aspek tersebut sebagai dasar bagi guru dalam merancang asesmen formatif dan pembelajaran matematika yang lebih responsif terhadap kondisi psikologis siswa. Mathematics anxiety is one of the factors that can affect students' ability to complete mathematics assessments. Studies on mathematics anxiety in the context of formative assessment remain limited, particularly those that comprehensively examine its emotional, cognitive, physiological, and behavioral aspects in the topic of polyhedrons, which requires students' spatial visualization and geometric reasoning abilities. This study aimed to analyze students' mathematics anxiety based on emotional, cognitive, physiological, and behavioral aspects during formative assessment on the topic of polyhedrons. The study employed a qualitative approach involving 35 seventh-grade students at SMP Muhammadiyah 5 Surakarta. Data were collected through a mathematics anxiety questionnaire, formative assessment tests, observations, and interviews. Based on the questionnaire results, students were classified into low, moderate, and high mathematics anxiety categories, and one student from each category was purposively selected for an in-depth analysis. Data were analyzed through data reduction, data display, and conclusion drawing, while data trustworthiness was established through technique triangulation. The findings revealed that students with low mathematics anxiety solved problems systematically and confidently, whereas those with moderate anxiety demonstrated hesitation in making mathematical decisions despite being able to complete the tasks. In contrast, students with high mathematics anxiety experienced difficulties in understanding problems, determining appropriate solution strategies, and completing calculations due to emotional, cognitive, physiological, and behavioral barriers. This study contributes to a comprehensive understanding of students' mathematics anxiety across these four dimensions and provides practical implications for designing formative assessments and mathematics instruction that are more responsive to students' psychological conditions.