The present research tests a model derived from self-determination theory to explain why self-concealment (the tendency to keep distressing personal information secret) is associated with negative well-being outcomes. Two studies tested a model in which self-concealment predicts the thwarting of basic needs for autonomy, competence, and relatedness, which then results in negative psychological outcomes. Study 1 involved a cross-sectional design. Structural equation modeling analyses revealed that the model provided an acceptable fit to the data. Study 2 involved a multilevel design. Participants completed daily measures of self-concealment, need satisfaction, and well-being over 16 days. Results supported the proposed mediation model. Furthermore, the associations between daily self-concealment, daily need satisfaction, and daily well-being were independent of trait self-concealment. Overall, the findings suggest that concealing personal distressing information is detrimental to the satisfaction of basic psychological needs, which in turn predicts negative well-being.