Seeds are crucial for plant population maintenance and dispersal. Invasive species often exhibit seed traits that enhance their colonization success, such as increased dispersal potential, earlier germination, or greater resource reserves. However, few studies have compared seed traits between invasive and native plant species along environmental gradients. Here, we compared morphological traits and nutrient concentrations of the seeds of two competing species, the native common reed (Phragmites australis) and the invasive saltmarsh cordgrass (Spartina alterniflora), along a 20° latitudinal gradient of the Chinese coast, and their relationships with environmental factors. Significant differences were found between the two species for 11 of the 13 traits with respect to latitude. Specifically, the seed size of S. alterniflora decreased with increasing latitude, but P. australis showed a slight curvilinear relationship with latitude, reducing to a minimum between 30 and 35° N. The latitudinal variation in seed set showed the opposite trends in both species at high latitudes. Seed nitrogen concentration decreased with latitude in both species, while seed phosphorus concentration declined only in P. australis. We also identified that temperature-dependent climatic variables were more important than soil properties in affecting the latitudinal variations of seed traits for both species, especially for S. alterniflora. Based on the results, we predict that the greater fecundity of S. alterniflora populations poses an increasing threat to P. australis at the higher latitudes as temperature rises due to climate change.