Societies that develop on islands in oceans, distant from continental shores and one another, are unusually vulnerable to fundamental change (collapse). It is argued that a common cause of such change is the effect of external (climate-driven) environmental forcing on food resources, especially those on which coastal-dwelling island peoples invariably depend. Relative changes in sea level that were comparatively rapid are implicated in several instances of societal collapse on islands; two examples are discussed. The first refers to western tropical Pacific Island groups in which Early Period societies are distinctive, representing periods of human settlement beginning 3,500–2,800 years ago and undergoing major transformative change a few centuries later. The end of these Early Period societies appears to have been near-synchronous, an observation requiring an external and region-wide driver rather than local drivers. Sea-level fall, which began 4,000–3,000 years ago in this region, continued for some centuries and is considered to have dropped below a critical threshold about 2,570 years ago, abruptly reducing useful coastal bioproductivity and forcing the inhabitants of these islands to sharply reduce their dependence on coastal foods. Second is the effect of sea-level fall during the AD 1300 Event (approximately AD 1250–1350) that rapidly reduced coastal food availability and resulted in conflict that forced human groups throughout the higher islands of the tropical Pacific to abandon coastal settlements in favour of those in defensible locations inland, upslope and offshore. On lower islands (atolls), people made use of islands newly formed as a result of the sea-level fall during the ad 1300 Event, while some islands were abandoned by people altogether. External environmental change, particularly sea-level change, has demonstrated potential to force fundamental alterations to island societies and even cause their collapse. This situation remains the same in today’s more globalised island world, where some islands are likely to become uninhabitable within a few decades as a result of sea-level rise, while on others deleterious impacts on coastal food systems are likely to force coastal peoples to seek new ways of feeding themselves.