Motivational interactions during multiple language learning have been largely neglected in language motivation research. To fill this gap, we investigate longitudinal relations between Swiss German students' value beliefs in English, French, and German in upper secondary schools and whether there are differences in motivational development between multilingual and monolingual students. Multivariate latent growth modeling was used to analyze data from 850 students (M[subscript age] = 15.61 years, SD = 0.62; 54% female) gathered yearly from Grades 9 to 11. Results suggest an interference between students' value beliefs in English and the other 2 languages. Students who reported higher value beliefs in English in Grade 9 showed steeper decreases in their value beliefs for French and German from Grades 9 to 11. However, stronger increases in English value beliefs over time were associated with stronger increases in French and German value beliefs. Moreover, while multilingual students reported higher initial value beliefs in French and English, they also showed steeper decreases in French and English value beliefs over time compared to their monolingual peers. Findings are discussed in relation to their implications for teaching practice and future research directions.