Abstract: This study aimed to produce a GeoGebra-based, Problem Based Learning (PBL)-integrated Student Worksheet (LKPD) that is valid, practical, and effective in strengthening mathematical concept comprehension among students, with a focus on Cartesian plane reflection transformation. A Research and Development (R&D) design was applied, utilizing the 4D framework (Define, Design, Develop, Disseminate) with implementation restricted to the Develop stage. The research involved 35 Grade XI students from SMA Negeri 1 Air Joman in the 2025/2026 academic year. Instruments comprised expert validation rubrics, questionnaires for teacher and student feedback, and a concept comprehension assessment. Validator ratings produced an LKPD validity score of 85% (Very Valid). Teacher and student practicality scores were 89% and 91.2% respectively (Very Practical). The mastery percentage from the effectiveness test reached 91% (Very Effective). It can thus be concluded that the GeoGebra-assisted PBL-based LKPD is worthy and effective as a mathematics teaching material on transformation topics. Keywords: LKPD, GeoGebra, Problem Based Learning, Mathematical Concept Understanding, Transformation. Abstrak: Tujuan dari penelitian ini adalah menghasilkan Lembar Kerja Peserta Didik (LKPD) berbantuan aplikasi GeoGebra berbasis Problem Based Learning (PBL) yang berkualitas valid, praktis, dan efektif dalam mendukung peningkatan kemampuan pemahaman konsep matematis siswa pada pokok bahasan transformasi pencerminan di koordinat Kartesius. Penelitian ini menggunakan rancangan Research and Development (R&D) dengan model 4D (Define, Design, Develop, Disseminate) yang pelaksanaannya dibatasi sampai fase Develop. Partisipan penelitian berjumlah 35 siswa kelas XI SMA Negeri 1 Air Joman Tahun Ajaran 2025/2026. Instrumen yang digunakan meliputi lembar penilaian validator, kuesioner tanggapan guru dan siswa, serta soal tes pemahaman konsep. Penilaian ahli menghasilkan kevalidan LKPD sebesar 85% (Sangat Valid). Kepraktisan LKPD tercatat 89% oleh guru dan 91,2% oleh siswa (Sangat Praktis). Tingkat ketuntasan pada uji keefektifan mencapai 91% (Sangat Efektif). Berdasarkan capaian tersebut, LKPD berbantuan GeoGebra berbasis PBL dinyatakan layak dan efektif untuk diimplementasikan sebagai bahan ajar matematika pada materi transformasi. Kata Kunci: LKPD, GeoGebra, Problem Based Learning, pemahaman konsep matematis, transformasi.