The study aims to investigate how confidence in mathematics/science, amount of books in the home, gender, whether a student likes being in school, and computer software predicted mathematics and science achievements of students in the Trends in International Mathematics and Science Study (TIMSS) 2019. The sample of the study consisted of eighth-grade Turkish students, 1784 students in mathematics, 1776 students in science, and 72 school principals. Multiple linear regression was conducted using the Statistical Package for the Social Sciences (SPSS) and International Database (IDB) Analyzer, which provides a way to combine five plausible values of achievement scores. The study results indicated that the amount of books in the home, whether the student liked being in school, and the students' confidence in mathematics/science significantly predicted mathematics and science achievements. Confidence in mathematics/science contributed the most to predicting achievement among the variables after controlling for the other predictors.