Objective Nonsuicidal self‐injury (NSSI) is a public health concern and risk factor for suicide. The Emotional Cascade Model (ECM) proposes that NSSI partially functions as a distraction from cascades of negative affect and rumination. The purpose of this study was to examine the roles of trait rumination, and momentary instability of rumination and negative emotion, in NSSI. Method Experience sampling methods were used to monitor thoughts, emotions, and behaviors in 47 individuals reporting dysregulated behaviors including NSSI. Instability indices were generated for rumination and negative emotion using the momentary assessments. Results Twenty‐five episodes of NSSI were reported during monitoring. Trait rumination prospectively predicted NSSI episodes, and the instability indices interacted to predict NSSI. Conclusions Consistent with the ECM, the interaction between rumination instability and negative affect instability during monitoring significantly predicted NSSI, with the strongest effects occurring for sadness and rumination about past. These findings may enhance conceptualization and treatment of patients with NSSI.