The primary aim of the current study was to examine the unique contribution of psychological need frustration and need satisfaction in the prediction of adults’ mental well-being and ill-being in a heterogeneous sample of adults (N= 334; M age= 43.33, SD= 32.26; 53% females). Prior to this, validity evidence was provided for the German version of the Basic Psychological Need Satisfaction and Frustration Scale (BPNSFS) based on Self-Determination Theory (SDT). The results of the validation analyses found the German BPNSFS to be a valid and reliable measurement. Further, structural equation modeling (SEM) showed that both need satisfaction and frustration yielded unique and opposing associations with well-being. Specifically, the dimension of psychological need frustration predicted adults’ ill-being. Future research should examine whether frustration of psychological needs is involved in the onset and maintenance of psychopathology (eg, major depressive disorder).