The high carbonate island of Niue has attracted the attention of scientists since the early European exploration of the South Pacific, but its extensive karst and numerous caves have so far received little attention. Our investigation recognizes two main types of caves on Niue: (1) steeply inclined, terraced, flank-margin caves that formed at the seaward edge of a migrating freshwater lens and that breach the vertical cliffs on the leeward side of the island, and (2) flat-roofed, water-table caves consisting of interconnected passages that developed at the paleo–water table and contain dissolution features caused by alternating vadose and phreatic conditions. The Niuean caves contain a large variety of both active and fossil speleothems, the forms and styles of which are controlled by the hydrodynamics of vadose and phreatic waters, the physical properties of the carbonate cap, and the tectono-eustatic history of the island. The stalagmites are composed exclusively of calcite and show pronounced laminations consisting of alternating light and dark couplets, likely representing austral summer and winter growth, respectively. The mean growth rate of stalagmites from coastal flank-margin caves (∼0.34 mm/yr) is faster by about a factor of 1.5 relative to stalagmites from inland water-table caves. In general, vadose and phreatic waters fall on the regional δ¹⁸O-δD meteoric water line, and their dissolved carbon is primarily derived from soil CO₂. A material balance suggests that cave deposits act as sinks for Mg, Sr, Na, and CO₂ during the recharge of the aquifer. The Niuean caves and their speleothems offer exceptional opportunities for investigating problems bearing on the regional tectono-eustatic history and paleoseismicity of the neighboring Tonga Trench, and answer questions concerning the prehistoric rhythms and irregularities of El Niño–Southern Oscillation.