Self-concept clarity and self-esteem are powerful determinants of people’s thoughts, feelings, and behaviors. Yet, even after over 30 years of research, the conceptual and empirical ties between these two self-aspects remain understudied, and little is known about the nature and function of their relationship. The present study aimed at discerning the empirical similarities and differences between self-concept clarity and self-esteem in order to examine whether they can be considered sibling constructs that are conceptually and empirically related, but not identical. Specifically, we used multivariate eight-wave longitudinal data from two cohorts of young adults (Cohort 1, N = 460; Cohort 2, N = 412) to examine the structural, cross-sectional, and longitudinal links between self-concept clarity and self-esteem, as well as their incremental validity and within-person transactions over time. We found strong evidence that self-concept clarity and self-esteem are not the same but sibling constructs. Specifically, both self-aspects were as follows: (a) structurally different and (b) cross-sectionally related, (c) shared similar but not identical developmental trajectories, (d) codeveloped over time, (e) demonstrated incremental validity in the prediction of important life outcomes, and (f) demonstrated distinctive within-person associations over time. Overall, the present study provides the most comprehensive demonstration that self-concept clarity and self-esteem are similar but different in several meaningful ways, which is of crucial importance for theory, research, and practice.