Background: Dynamic, relational developmental systems-based models of development emphasize that developmentally-nurturant youth-adult relationships elicit in youth perceptions of being known and loved. Although such perceptions are foundations of positive youth development (PYD), such measures do not exist. Objective: We sought to create a theoretically-predicated measure of youth perceptions of being known and loved by capitalizing on data sets in two countries (Rwanda and El Salvador) wherein a multi-national study of PYD was being conducted by Compassion International (CI). Method: With Rwanda data (n = 1,204, M[subscript age] = 11.84, 50% CI-supported), exploratory and confirmatory factor analyses enabled refining the measure for robustness and parsimony. Measures of intentional self-regulation, hopeful future expectations, transcendence, and contribution were used for validation of the known and loved measure within the nomological net of constructs proposed in the Lerner and Lerner PYD model. Confirmatory factor analyses supported the use of the model within the El Salvador data set (n = 1,205, M[subscript age] = 13.03, 51% CI-supported). Results: Robust psychometric properties were established in both national settings. Measurement invariance was found across age, gender, urban--rural location, CI-enrollment status, and nations, and involved both mean differences and correlations among latent factors. Conclusions: The results provide evidence for a theory-predicated measure of youth perceptions of being known and loved and that scores for this construct covary within a nomological net specified in the Lerner and Lerner model of PYD. These findings serve international development organizations seeking theory-predicated measures for use in evaluating PYD programs in low- and middle-income countries.