The Isthmus of Panama illustrates how the vegetation of a newly created landscape in a tropical setting evolves over time. It also allows us to investigate biological invasions, because the landscape was first connected to temperate North America, and later connected to tropical South America. Using a large number of outcrops newly exposed during the recent expansion of the Panama Canal, we were able to complement the extensive palynological research that Alan Graham conducted in Panama over the past 25 years. We analyzed the palynological record of the interval 19.5–1.2 Ma, represented by 282 samples containing 27,910 grains (pollen/spores) with 496 morphotypes. Further, a revision of the plant macrofossil literature of Panama and analysis of the carbon isotope content of 14 samples were carried out. Our results indicate that since the Early Miocene, Panamanian forests have been dominated by Gondwana-Amazonian taxa, suggesting that plants were able to cross the Central American Seaway much earlier than mammals. The landscape was dominated by tropical rainforest and lower montane to montane forest, contrary to the dry and open habitats that some previous studies have proposed. Plant diversity seems to have increased over the past 10 My, but it is unclear if this increase is due to a taphonomic bias. Further studies are needed to understand the relationships of the Early Miocene Panamanian mammals derived from North American temperate forest lineages as they faced new habitats in Panama dominated by South American–derived tropical rainforest.