This article reports the results of a 33-wave longitudinal study of relations between job insecurity and physical and mental health based on monthly data collected between April 2020 and December 2022 among n = 1,666 employees in Germany. We integrate dynamic theorizing from the transactional stress model and domain-specific theorizing based on stressor creation and perception to frame hypotheses regarding dynamic and reciprocal relations between job insecurity and health over time. We find that lower physical health predicted subsequent increases in job insecurity and higher physical health predicted subsequent decreases in job insecurity. However, job insecurity did not have a significant influence on physical health. Furthermore, higher job insecurity predicted subsequent decreases in mental health, and higher mental health predicted subsequent decreases in job insecurity. This pattern of findings suggests a dynamic and reciprocal within-person process wherein positive deviations from one's average trajectory of job insecurity are associated with subsequently lower levels of mental health and vice versa. We additionally find evidence for linear trends in these within-person processes themselves, suggesting that the strength of the within-person influence of job insecurity on mental health becomes more strongly negative over time (i.e., a negative amplifying cycle). This research provides practical insights into job insecurity as a health threat and shows how concerns about job loss following deteriorations in physical and mental health serve to further threaten well-being. (PsycInfo Database Record (c) 2024 APA, all rights reserved).