<![CDATA[Abstract: The aim of this research is to find out whether or not there is significant correlation between social media usage and English vocabulary mastery of the fifth-semester students of the English Study Program Universitas Riau. By applying the quantitative method, this research used a population with the total 112 students of the fifth semester, while the samples were 76 students, who were selected using a cluster random sampling technique. The research isntruments are a Questionnaire and a Test. The questionnaire is to measure social media usage (Safitri, 2021; Sivagnanam and Yunus, 2020), and the vocabulary test is to measure the vocabulary mastery of the students adapted from TOEFL book (Riau University Language Center, 2014) and TOEFL book by (Kardimin, 2008). The researcher used IBM SPSS Statistics 25 to analyze the data. The results of the data analysis showed that the average student's use of social media is 92.3, which is considered to be at a "high" level, while the average student's mastery of vocabulary is 79.4, which is considered to be at a "good" level. Additionally, the research finding showed that the correlation coefficient of the two variables is 0.489, considered a positive and moderate correlation. and the determinant coefficient is 0.2391, which implies social media usage has a 24% influence on the students' vocabulary mastery, meanwhile 76% being influenced by other factors.Keywords: Correlation, Social Media Usage, Vocabulary Mastery]]>
