Students’ awareness and attitudes toward YouTube influence its use in mathematics learning. This study examined the relationship between students’ knowledge, attitude, and practice in using YouTube as a supplementary learning tool among Grade 10 students. A total of 186 students from Dulag National High School, selected through stratified random sampling, participated in the data collection. Using descriptive analysis, the findings revealed that students have a very high level of knowledge (M = 4.23), a positive attitude (M = 3.98) and often practice (M = 3.77) using YouTube as a supplementary learning tool in mathematics. Using correlation analysis, the result showed a moderate, positive, and significant relationship between students’ knowledge and practice (r = 0.534, p < 0.001), suggesting that a higher level of knowledge is positively associated with more frequent use of YouTube as a supplementary learning tool in mathematics. Furthermore, a moderately strong, positive, and significant relationship was found between students’ attitude and practice (r = 0.677, p < 0.001), indicating that students with a more positive attitude are more likely to use YouTube often as a supplementary learning tool in mathematics. The findings suggest that educators and school should provide activities or programs that enhance students’ knowledge of YouTube and fosters a positive attitude towards its educational use.