Local adaptation and phenotypic plasticity are two alternative mechanisms used by invasive plants for range expansion. We conducted a series of experiments to investigate the role of these mechanisms in the recent expansion of the invasive Ipomoea cairica from non-saline to salt-stressed coastal habitats. A comparison of the plant’s photosynthetic traits and construction costs across habitats was conducted through a field survey. Meanwhile a full factorial greenhouse experiment was conducted with two ecotypes (non-saline and coastal) of I. cairica and two salinity gradients (water and 4 g L-1 NaCl solution) to evaluate the roles of the two strategies by comparing their main traits. The results revealed that the construction cost and Amax of I. cairica did not change with the habitat type. The ecotype and saline treatments however significantly influenced the plant growth. The non-saline ecotype (NE) generally showed higher or equal plasticity of biomass-allocation and functional traits compared to the coastal ecotype (CE). However the fitness and biomass of the NE significantly decreased with salinity whereas those aspects of the CE did not change. Our results indicate that the recent expansion of I. cairica into coastal areas may be accelerated by the local adaptation of the CE to salt stress. Additionally in South China the CE will most likely evolve adaptations to both saline and non-saline environments which will further broaden the invasion range of I. cairica in the future.