This study aims to determine the validity and practicality of the product developed in the form of a mathematics learning device based on Realistic Mathematical Education (RME) in the flat-sided geometry material for Package B students equivalent to class VIII SMP. This research was conducted at the Kasih Bundo Community Learning Activity Center, Bukittinggi. This research is a development research using the Plomp development model. The results of the research discussed are related to the results of the validity test and practicality test. The validity test is based on the results of the expert review, while the practicality is based on one to one and small group trials in the field. The validity test obtained very valid criteria, while for the one to one and small group activities it was found that there was an increase after using the student worksheet (LKPD) which was developed using the RME approach.Penelitian ini bertujuan untuk mengetahui validitas dan praktikalitas dari produk yang dikembangkan berupa perangkat pem,belajaran matematika berbasis Realistic Mathematic Education (RME) pada materi bangun ruang sisi datar untuk siswa Paket B setara kelas VIII SMP. Penelitian ini dilakukukan di Pusat Kegiatan Belajar Masyarakat Kasih Bundo Bukittinggi. Penelitian merupakan penelitian pengembangan menggunakan model pengembangan Plomp. Hasil penelitian yang dibahas terkait hasil uji validitas dan uji praktikalitas. Uji validitas berdasarkan hasil expert review sedangkan praktikalitas berdasarkan uji coba one to one dan small group di lapangan. Uji validitas tersebut memperoleh kriteria sangat valid, sedangkan untuk kegiatan one to one dan small group diketahui adanya peningkatan setelah menggunakan lembar kerja peserta didik (LKPD) yang dikembangkan menggunakan pendekatan RME.