SUMMARY
Vocabulary mastery is a crucial skill for English language learners. It will be much simpler for students to communicate verbally and in writing if they have a strong command of the English language's vocabulary. Considering this issue, this research aims to investigate the correlation between how often Senior High School students watch English content on YouTube and how well they know their vocabulary. In this research, the method is quantitative and the design is a correlation. This research population was eleventh graders of a Senior High School in Sidoarjo, while the sample was 45 students majoring in science. The first data collection instrument was a questionnaire used to collect data on student habit of watching English YouTube content. The second instrument was a vocabulary test used to collect data on students’ vocabulary mastery. Both results of data collection were analyzed and calculated using Microsoft Excel 2013 and SPSS version 26. This research found that the r-count was higher than the r-table. The r-count was 0.528 while the r-table was 0.294. It indicated a significant correlation between the two variables. The more often the students watch English YouTube content, the greater their vocabulary mastery. Then, H1 is accepted and H0 is rejected.