Penelitian ini bertujuan untuk mengembangkan bahan ajar berupa Lembar Kerja Peserta Didik (LKPD) Matematika berbasis problem based learning pada materi sistem persamaan linier tiga variabel (SLTV) kelas X. Kualitas produk yang dikembangkan dinilai berdasarkan aspek kevalidan, kepraktisan dan keefektifan. Prosedur pengembangan LKPD mengacu pada model pengembangan atau Research and Development (R&D) denga pendekatan desain system pembelajaran yaitu model 4-D dimodifikasi menjadi 3-D dikarenakan keterbatasan peneliti, penelitian dibatasi hingga tahap develop (pengembangan) saja. Subjek penelitian ini adalah siswa kelas X MAS Plus Al-Ulum Medan, berjumlah 36 siswa. Hasil penelitian menunjukkan : (1) LKPD yang dikembangkan sangat valid/sangat layak digunakan dengan indeks Aiken yang diperoleh untuk media LKPD sebesar 0,92 untuk ahli materi, 0,84 untuk ahli strategi, dan 0,86 untuk ahli bahasa dengan kategori untuk semua penilaian tinggi atau sangat valid. Dan diperoleh indeks sebesar 0,94 oleh validator ahli media dengan kategori tinggi/sangat baik oleh ahli medi. (2) LKPD yang dikembangakn praktis digunakan berdasarkan respon guru yang memperoleh rata-rata presentase 94,3% dengan kriteria “sangat praktis”. (3) LKPD dikatakan keefektifan dengan Presentase ketuntasan klasil peserta didik diperoleh dengan rata-rata sebesar 88,8% dengan kategori sangat baik/ efektif digunakan. Kata Kunci : LKPD; 4D; problem based learning. ABSTRACT This study aims to develop teaching materials in the form of Problem Based Learning Mathematics Student Worksheets (LKPD) in class X three-variable linear equation system (SLTV) material. The quality of the products developed are assessed based on aspects of validity, practicality and effectiveness. The LKPD development procedure refers to the development model or Research and Development (R&D) with a learning system design approach, namely the 4-D model modified to 3-D due to the limitations of the researcher, the research is limited to the develop stage only. The subjects of this study were students of class X MAS Plus Al-Ulum Medan, totaling 36 students. The results showed: (1) the worksheets developed were very valid/very feasible to use with the Aiken index obtained for LKPD media of 0.92 for material experts, 0.84 for strategists, and 0.86 for linguists with categories for all high or very valid rating. And obtained an index of 0.94 by media expert validators with a high/very good category by medical experts. (2) Practically developed LKPD is used based on the teacher's response who obtained an average percentage of 94.3% with the criteria of "very practical". (3) LKPD is said to be effective with the percentage of classical completeness of students obtained with an average of 88.8% with a very good / effective category used. Keywords : LKPD; 4D; Problem Based Learning.