The giant Amazonian waterlily (genus Victoria) produces the largest floating leaves of all plants. We studied how the structural form of the vasculature system underpins gigantism in these extraordinary leaves. and infer how this unique form of leaf gigantism evolved. Specifically, by means of mechanical testing and geometrical modelling, we found that the bending resistance of the Amazonian waterlily is considerably higher than of an elastic floating sheet of the same amount of material. Our analysis suggests that the unique pattern of branching veins on the underside of the Victoria leaf provides structural support at an economical material cost and, as such, enables gigantism. Considering the evolutionary history of the waterlily family, and the life history of Victoria in particular, we inferred that this multi-purpose system may have evolved to maximise photosynthesis and enable rapid growth in fast-drying pools, thereby conferring a selective advantage in an unstable environment.