["Applied Psychology: Health and Well-Being, Volume 18, Issue 4, August 2026. ", "\nAbstract\nThis research provides the first quasi‐experimental and longitudinal examination of the Social Identity Model of Traumatic Identity Change among individuals affected by the September 2024 flood in Poland. Advancing the state of the art, we analyzed the emergence of identification with flood‐affected people and extended the model by incorporating the sense of shared experience (SSE) within this group. Surveys were completed 3 months (N = 630) and 7 months after the flood (N = 315).\nThe results indicated that the severity of flood exposure was linked with higher post‐traumatic stress (PTS) and post‐traumatic growth (PTG). Repeated trauma strengthened both identification and SSE with those affected by the flood. Over time, SSE remained stable, whereas identification decreased, suggesting that SSE serves as a more enduring psychosocial resource than group identification. SSE predicted the subsequent PTG but not the PTS, highlighting its positive role in adaptive coping, while PTS contributed to later increases in SSE. Multi‐group analyses comparing participants from flood‐prone areas who were at risk but had not experienced flooding, those with less recent flood exposure, those with recent flood exposure, and those experiencing re‐traumatization from both past and recent floods revealed that the pathways linking identification, SSE, and post‐traumatic outcomes differed across these groups. These findings demonstrate the differential function of social processes under varying trauma contexts. Overall, this research underscores the importance of considering exposure history in post‐disaster interventions and highlights how social identity and shared experience can both support resilience and shape recovery trajectories.\n"]