Tropical cyclone (TC) rapid intensification (RI) is a major source of uncertainty in TC prediction. Here we examine observed basin-specific relationships between RI and El Niño-Southern Oscillation (ENSO), where RI is defined as a TC strengthening by >=30 kt within 24 hr. During El Niño, there is a significant increase in RI in the eastern North Pacific, western North Pacific and South Pacific, with the opposite behavior in the North Atlantic. During La Niña, RI has an approximately opposite relationship in the Atlantic and Pacific. The relationship between ENSO and Indian Ocean RI is weak. These changes are closely linked to environmental conditions modulating RI, including mid-level moisture, vertical wind shear and sea surface temperatures. Because there is disagreement between the recently-observed La Nina-like trend and an El Niño-like trend simulated by climate models, anticipating future RI trends is conditional on improved model representation and physical understanding of ENSO.