Perfectionism cognitions capture automatic perfectionistic thoughts and have explained variance in psychological adjustment and maladjustment beyond trait perfectionism. The aim of the present research was to investigate whether a multidimensional assessment of perfectionism cognitions has advantages over a unidimensional assessment. To this aim, we examined in a sample of 324 university students how the Perfectionism Cognitions Inventory (PCI) and the Multidimensional Perfectionism Cognitions Inventory (MPCI) explained variance in positive affect, negative affect, and depressive symptoms when factor or subscale scores were used as predictors compared to total scores. Results showed that a multidimensional assessment (PCI factor scores, MPCI subscale scores) explained more variance than a unidimensional assessment (PCI and MPCI total scores) because, when the different dimensions were entered simultaneously as predictors, perfectionistic strivings cognitions and perfectionistic concerns cognitions acted as mutual suppressors thereby increasing each others’ predictive validity. With this, the present findings provide evidence that—regardless of whether the PCI or the MPCI is used—a multidimensional assessment of perfectionism cognitions has advantages over a unidimensional assessment in explaining variance in psychological adjustment and maladjustment.