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Mangrove forests are increasingly threatened by plant invasions worldwide, but some mangrove species are invasive and threaten salt marsh and native mangrove ecosystems. The southern coast of China is invaded by the cordgrass <i>Spartina alterniflora</i>, and the mangrove <i>Sonneratia apetala</i>, providing a model system for studying the processes and mechanisms through which non-native species establish and spread. We used a transplant experiment to test the overarching hypothesis that native herbivores and plants provided biotic resistance against invasion by <i>S. apetala</i>, and that the importance of these factors would vary geographically. Survival of transplanted mangrove seedlings was lower in Zhangjiang Estuary (23°55′ N) than in Leizhou Bay (20°56′ N), and varied with species and habitats. <i>S. apetala</i> had higher survival and growth rates than native mangroves at both sites, and <i>S. apetala</i> grew taller than the <i>S. alterniflora</i> canopy at Leizhou Bay in only two growing seasons. In contrast, native mangroves grew poorly in <i>S. alterniflora</i>. Grazing by rodents and insects suppressed the growth and survival of <i>Kandelia obovata</i> and <i>Avicennia marina</i> in Zhangjiang Estuary and Leizhou Bay, but had little effect on <i>S. apetala</i>. Competition with vegetation (<i>S. alterniflora</i> and native mangroves) exacerbated the reduced survival of native mangroves, and these effects varied across study sites. Low survival of non-native <i>S. apetala</i> in vegetated habitats at Zhangjiang Estuary was likely due to a synergistic effect of low winter temperatures and low light intensity. Escape from herbivory (the opposite to biotic resistance) and fast growth may drive the quick expansion of non-native <i>S. apetala</i> in China. Rapid encroachment of <i>S. apetala</i> may transform the native mangrove forests and monospecific intertidal <i>Spartina</i> grasslands into non-native mangrove forests in the southern coast of China.