The study of character becomes a very important discussion in the 21st century. So that the integration of character values is very important both in the process and in educational assessment. The purpose of this study was to test the validity and reliability of the character assessment instrument for 21st-century high school students. The research approach used was quantitative with a sample of 200 high school students. Data analysis carried out includes validity and reliability tests. The test results of the instrument showed that the construct of the student character assessment instrument was declared valid and reliable. The content validity test shows the value of Aiken's > 0.80 in the high category. In the construct validation test with EFA, all variables have a loading factor > 0.5. In the CFA test, the model is declared fit with the estimated standard loading value of 0.40 and the t-count value > 1.96. Meanwhile, while in testing the reliability of the instrument obtained composite > 0.70 Cronbach's Alpha reliability > 0.70 which means reliable. So that this instrument is declared valid and reliable to measure the character of students in high school.