Wedelia trilobata (L.) Hitchc., a creeping herb that originated from Central and South America, is listed as one of the world’s 100 worst invasive alien species. After its introduction to south China, it has become a common invasive plant in cropland, plantations and natural forests. Wedelia chinensis is the native congener of W. trilobata in China, but has not been found harmful to native plants or habitats. The aims of this study were to investigate the effects of drought stress and rehydration on chlorophyll fluorescence, antioxidative activity and some other ecophysiological parameters in leaves of the alien invasive W. trilobata and its native congener W. chinensis, and explore the response and ecological adaptability of this invasive species to drought stress. Soil water content during the course of natural drought treatment of W. trilobata plants decreased faster than in the case of W. chinensis plants. A comparison of stomatal morphology revealed that the opening of stomata was remarkably greater in leaves of W. trilobata than that of W. chinensis after 11d of drought stress, which may partly explain the faster water loss in W. trilobata. After 11d of drought stress, W. trilobata showed a significant decrease of 43.8% in maximum quantum efficiency of photosystem Ⅱ photochemistry (Fv/Fm), compared with a much smaller decrease of 3.7% in W. chinensis. In comparison with W. chinensis, greater decreases of the effective PSⅡ quantum yield (Yield), apparent electron transport rate (ETR) and coefficient of photochemical quenching (qP) were also found in W. trilobata by the end of drought treatment. These observations indicate that this invasive species is susceptible to drought stress. However, the alterations in W. trilobata could rapidly recover after rehydration. Increased heat dissipation of excess excitation energy and enhanced the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) for scavenging oxygen radicals alleviated irreversible damage of the photosynthetic apparatus during drought stress and promoted recovery when the drought stress was released. In conclusion, W. trilobata is susceptible to drought stress and its spread to arid regions is likely to be limited.