Mangrove forests are characterized by highly stressful conditions and are thought to be resistant to plant invasions (the ‘mangrove invasion resistance paradigm’). Nevertheless, the exotic mangrove Sonneratia apetala is encroaching into the native shrubby mangroves and invasive salt marshes (Spartina alterniflora) in southern China, forming a widespread marsh-mangrove ecotone. The mechanisms driving the rapid invasion of S. apetala remain unknown. Using a trait-based approach, we compared the trait-environment interactions between invasive S. apetala and native Avicennia marina through field transplant experiments and greenhouse shading trials. We found (1) S. apetala exhibited 5–10 times higher relative growth rate than A. marina across light gradients; (2) S. apetala achieved over 50% establishment in native shrubby mangrove stands and S. alterniflora meadows vs. zero for A. marina, while establishment of both species was zero in closed-canopy tree stands; (3) A trait syndrome combining with fast growth capacity, acquisitive leaf traits (higher photosynthetic rate and specific leaf area, and shorter leaf lifespan), and shoot-biased biomass allocation linked light availability to establishment of S. apetala, whereas A. marina’s conservative traits (higher leaf dry mass content and longer leaf lifespan) and root-biased biomass allocation decoupled from growth and establishment. S. apetala’s trait syndrome, which maximizes whole-plant growth in low-light shrubby mangrove and S. alterniflora meadow habitats, challenges the standard mangrove invasion resistance paradigm. Furthermore, multi-dimensional trait-environment-performance interactions may underlie the success of highly invasive species worldwide. Our results suggest that management priorities should be removing S. apetala seedlings before they reach escape height and protecting intact native mangrove vegetation.