In the Neogene, the western Amazon was extensively influenced by marine incursions. This is evident from the fossil record, which includes mangrove pollen, dinoflagellate cysts and microforaminiferal organic linings. Mangroves were thought to be populated only by members of the Rhizophoraceae, although Florschuetzia-type pollen have been also observed in cored sediments from westernmost Brazil. This raises the question as to whether sonneratioid mangroves of Southeast Asian origin extended into the Amazon. To investigate this question, we reanalysed the Brazilian core 1AS-4a-AM and applied light and scanning electron microscopy to selected sporomorphs. We also revised the temporal range of Florschuetzia occurrences and reassessed the morphological description of Florschuetzia impostora by applying confocal microscopy. Based on this study, we extend the description of F. impostora (D'Apolito et al., 2021) and present a new species, Florschuetzia amazonica sp. nov. Both species strongly resemble F. trilobata and F. ovalis, described from Southeast Asia, but their specific morphological traits, disjunct distribution and short-spanned abundant occurrences differentiate them. In the Amazon, Florschuetzia mangroves occured around 18 Ma and were associated with a high abundance of palm and mangrove-associated sporomorphs (pollen and spores), suggesting that they occurred in the back-mangrove zone, landward of the Rhizophoraceae belt. Extinct Amazonian long-snouted gavialoid crocodiles and river dolphins also resemble Southeast Asian taxa, but paleobiogeographic revision shows they are not related. Although multiple migration pathways were possible, we favor a Pacific route for the Asian sonneratioid mangrove lineages. This Early Miocene (or older) dispersal event contributed to a richer and more complex mangrove ecosystem than is seen in the region today.