Identifying sedimentary signatures of tsunamis within long-term records (hundreds to thousands of years) is time and labour intensive using existing techniques. The lack of rapid identification restricts our ability to reconstruct past (palaeo-) tsunamis on the large spatial and temporal scales necessary for accurate hazard and risk assessment needed by vulnerable, low-lying populations. Here we pioneer the use of portable optically stimulated luminescence (POSL) in rapidly assessing in the field the occurrence and imprint of palaeotsunamis in sedimentary records. Pescadero Marsh (California) has a 2000-year sedimentary record of high-energy events constrained by a robust chronology derived from multiple techniques (luminescence dating, pollution markers). Tsunamis on the Californian coast rework nearshore sediments that have well reset luminescence signals. In slowly-accreting settings with well reset baseline signals, it is difficult to identify tsunamis remobilising locally-derived, well-reset sediments. However, tsunami signatures are readily detectable under rapidly-accreting, back-barrier saltmarsh conditions where they contrast the intercalating, poorly-reset luminescence signals of the marsh sediments deposited under tidal and fluvial conditions. Thus, POSL can allow for rapid, in-the-field assessment of tsunami deposits because their luminescence properties differ from the baseline sedimentary signal. POSL also documents changes in environmental conditions arising from a tsunami event, such as marsh recovery after an erosional and/or co-seismic subsidence event. Where there is a contrast in properties of the tsunami signatures and the baseline sediments, the rapid POSL assessment will allow for landscape-wide datasets to be obtained that can fully capture tsunami signatures globally as the minerals used for measurements are almost ubiquitous on Earth.