Abstrak: Bahan ajar perlu dikembangkan untuk membantu guru dan siswa dalam proses pembelajaran. Penelitian ini bertujuan untuk mengetahui tingkat validitas dari pengembangan bahan ajar tematik berbasis Project Based Learning (PjBL) untuk siswa kelas IV sekolah dasar. Jenis penelitian ini merupakan penelitian pengembangan (R&D) dengan menggunakan model ADDIE. Data kuantitatif diperoleh dari hasil penilaian produk oleh validator. Data kualitatif diperoleh dari saran, catatan oleh validator. Teknik Pengumpulan data menggunakan wawancara, angket, dan dokumentasi. Hasil dari penelitian ini berdasarkan tingkat validitas bahan ajar dengan hasil sebagai berikut: (1) validasi ahli desain bahan ajar memperoleh nilai rata-rata 4,77 dengan kriteria “sangat valid”, (2) validasi ahli pembelajaran bahan ajar memperoleh nilai rata-rata  4,62 dengan kriteria “sangat valid”, (3) validasi ahli kebahasaan bahan ajar memperoleh nilai rata-rata 4,85 dengan kriteria “sangat valid”. Sehingga dapat disimpulkan bahwa bahan ajar berbasis Project Based Learning (PjBL) pada pembelajaran tematik tema 6 subtema 3 kelas IV layak diuji coba kepada siswa dan guru.Abstract: Teaching materials need to be developed to assist teachers’ and students’ learning process. This study aims to measure the level of validity of the development of thematic teaching materials based on Project-based Learning (PjBL) for fourth grade elementary school students. A research and development design (R&D) was selected, using the ADDIE model. While quantitative data were obtained from the expert validation results, qualitative data were obtained from validators’ feedback. Interview protocols, questionnaires, and documentation were used to obtain the data. The results of this study regarding the level of validity of the teaching materials showed that (1) the expert validation of teaching materials design obtained an average value 4.77 with the criterion "very valid"; (2) the expert validation of teaching materials learning obtained an average value 4.62 with the criterion "very valid"; and (3) the linguistic expert validation of teaching materials obtained an average value 4.85 with the criterion "very valid". Hence, it can be concluded that the teaching materials based on Project-based Learning (PjBL) in thematic learning for fourth grade elementary school students could be piloted to the teachers and students.