The uncontrolled range expansion of invasive plant species has become a worldwide problem in this century. Finding suitable biological control measures for these invasive species has become a focus for many biologists. Using biological control species that are not native to the invaded community can generate further problems in the community. Thus, in recent years, scientists have started to look for enemies that are (1) able to inhibit growth in an invasive species, and (2) native to the invaded region. Parasitic plants are one example of such novel native enemies. Several studies provide support for the use of native parasitic plants as potential biological control agents for invasive plants. However, the exact response of invasive plants to the parasitic plants is still poorly known. In this study, the relationship between a common invasive plant Alternanthera philoxeroides (Amaranthaceae) and its native parasitic plant Cuscuta australis (Convolvulaceae) was investigated. A. philoxeroides is a notorious invasive weed that originates from South America. It is widely distributed in China and has had a marked negative impact on local biodiversity and the economy of China. C. australis, a native holoparasitic plant in China, has been found to naturally parasitize invasive A. philoxeroides in the field. The growth of the parasite depends on assimilation of the host's nutrients and water. A field survey was conducted to investigate the trade-off between growth and defense in A. philoxeroides in response to parasitization by C. australis . Biomass of A. philoxeroides and the secondary metabolite composition in the stems with or without C. australis were measured. Results showed the following: (1) Root mass, stem mass, leaf mass, total biomass and leaf number of A. philoxeroides were significantly reduced in plants parasitized by C. australis, but the number of stem nodes significantly increased. Clonal reproduction of A. philoxeroides mainly relies on the stem nodes; thus, the results indicate a significant inhibition of growth and an increased investment in clonal reproductive ability. (2) Parasitization by C. australis significantly increased the secondary metabolite contents in stems of A. philoxeroides, including lignin, total phenols, tannins and tri-terpenoid saponin. These four types of secondary metabolite play important roles in the plant when under stress. The increase in secondary metabolites indicates an enhanced defense capability in the host. (3) The total biomass of A. philoxeroides parasitized by C. australis was significantly reduced, while the relative percentage content of secondary metabolites significantly increased. Moreover, a significant negative correlation was found between total biomass and secondary metabolite content in stems of A. philoxeroides parasitized by C. australis. Overall, this study suggests that in response to parasitism by C. australis, A. philoxeroides alters its balance in investment between growth and defense, with less investment in growth and more on clonal reproduction and defense. Such a trade-off strategy between growth and defense may help invasive plants mitigate the negative effects of new native enemies in the invaded community. Nonetheless, native parasitic plants do provide a novel enemy for invasive plant species and may provide a less risky but effective way to control invasive plants.