The western tropical Pacific (WTP) is a global hotspot for sea-level rise (SLR) and intense climatic extremes, rendering low-lying islands acutely vulnerable to catastrophic extreme sea levels (ESLs) and inundation. However, the relative contributions and joint modulation of multi-scale drivers for ESLs remain poorly constrained, limiting our ability to assess future risk probabilities in this region. Here we show that a distinct spatial contrast in ESL forcing emerges in the WTP: while tides and weather extremes dominate in marginal seas, interannual-to-decadal sea-level variability governs the low-frequency modulation of ESL occurrences across WTP islands. Under compound conditions of strong La Niña and high tides, an SLR of merely 13–20 cm is likely (>83% probability) to trigger ESL events exceeding the 99.5^th percentile for WTP islands. Furthermore, while El Niño has historically buffered islands against ESLs by suppressing local sea levels, a 39–50 cm SLR would lead to a high probability (>83%) of ESL occurrence under high tides even during strong El Niño phases, effectively negating this climatic buffering effect. With observed SLR already approaching 10 cm at several island stations, our findings indicate that WTP islands are entering a regime of rapidly escalating inundation risk, driven by the synergistic interaction between persistent SLR and natural climate variability.