In this article, the authors used a within-person design to examine the relationship between job insecurity and employee in-role and extra-role performance, and the buffering role of time-varying work-based support (i.e., supervisor and colleague support) in this relationship. Weekly diary data gathered over the course of three weeks from 56 employees confronted with organizational restructuring and analyzed with a hierarchical linear modeling approach showed that weekly fluctuations in job insecurity negatively predicted week-level in-role performance. As predicted, supervisor support moderated the intra-individual relationship between job insecurity and in-role performance, so that employees' in-role performance suffered less from feeling job insecurity during weeks in which they received more support from their supervisor. No relationship between job insecurity and extra-role performance was observed. This within-person study contributes to research on job insecurity that has primarily focused on inter-individual differences in job insecurity and their associations with job performance. Theoretical and practical implications for human resource management are discussed.