Penelitian ini bertujuan untuk mengetahui dan menganalisis tentang pengaruh social media usage dan electronic word of mouth terhadap purchase desicion involvement yang dimediasi oleh trust dengan objek penelitian BliBli. Penelitian yang penulis digunakan adalah basic research yang menggunakan empat variabel yaitu social media usage, electronic word of mouth, trust, dan purchase decision involvement. Sumber data yang digunakan adalah data primer yaitu dengan menyebarkan kuesioner secara online. Sampel yang digunakan dalam penelitian ini 200 kuesioner. Data akan danalisa menggunakan SEM (Structural Equation Modeling) dengan program SmartPLS (Partial Least Square) versi 3.2.9 dan Microsoft Excell sebagai alat bantu. Hasil penelitian ini menunjukan social media usage dan electronic word of word memiliki pengaruh positif dan signifikan terhadap trust maupun purchased decision involvement. Trust juga memiliki pengaruh positif dan signifikan terhadap purchased decision involvement di BliBli. / This study aims to determine and analyze the influence of social media usage and electronic word of mouth on purchase desicion involvement mediated by trust with the object of research BliBli. The research that the author uses is basic research that uses four variables, namely social media usage, electronic word of mouth, trust, and purchase decision involvement. The data source used is primary data, namely by distributing online questionnaires. The sample used in this study was 200 questionnaires. The data will be analyzed using SEM (Structural Equation Modeling) with the SmartPLS (Partial Least Square) version 3.2.9 program and Microsoft Excel as a tool. The results of this study indicate that social media usage and electronic word of word have a positive and significant influence on trust and purchased decision involvement. Trust also has a positive and significant influence on purchased decision involvement in BliBli.