ABSTRACT Introduction Conspiracy mentality has been associated with various detrimental societal outcomes. However, research on its predictors and consequences has been focused almost exclusively on adults. Guided by Self‐Determination Theory, this study examined reciprocal associations between basic psychological need frustration and conspiracy mentality among adolescents. Methods A sample of 232 high school students ( M age = 14.6, SD = 1.53; 58% identifying as girls) completed surveys at two time points over a 1‐year interval (Spring 2023 and 2024) in the province of Quebec, Canada. Results Results indicated that frustration of the need for relatedness predicted an increase in conspiracy mentality, whereas conspiracy mentality predicted an increase in relatedness frustration, with small effect sizes. Moreover, conspiracy mentality predicted an increase in frustration of the need for autonomy, with a small effect size. Conclusion Given the negative implications of conspiracy beliefs, schools should implement strategies to address these beliefs and provide adolescents with opportunities to satisfy their needs for autonomy and relatedness.