Diploid cells undergoing senescence and mitotic slippage have been reported in the literature. However, the mechanisms triggering senescence in long-term G₂-arrested cells are currently unclear. Previously, we reported that the cell cycle of the human uveal melanoma cell line, 92-1, is suspended for up to 6 d upon exposure to 10 Gy ionizing radiation (IR), followed by senescence. In the current study, we initially distinguished senescence in long-term blocked 92-1 cells from mitotic slippage by confirming the blockage of cells in the G₂ phase. We subsequently showed that the genes essential for G₂-M transition are prematurely downregulated at both the transcriptional and translational levels. Furthermore, levels of the G₁-specific markers, Cyclin D1 and Caveolin-1, were distinctly increased, while S/G₂-specific markers, Cyclin B1 and Aurora A, were significantly downregulated. These findings collectively imply that long-term G₂-arrested cells undergo senescence via G₂ slippage. To our knowledge, this is the first study to report that the cellular process of G₂ slippage is the mechanism responsible for senescence of cells under long-term G₂ arrest.