There is a considerable controversy about whether western Amazonia was ever covered by marine waters during the Miocene [23 to 5 Ma (million years ago)]. We investigated the possible occurrence of Miocene marine incursions in the Llanos and Amazonas/Solimoes basins, using sedimentological and palynological data fromtwo sediment cores taken in eastern Colombia and northwestern Brazil together with seismic information. We observed two distinctmarine intervals in the Llanos Basin, an early Miocene that lasted similar to 0.9My (million years) (18.1 to 17.2 Ma) and amiddleMiocene that lasted similar to 3.7 My (16.1 to 12.4Ma). These twomarine intervals are also seen in Amazonas/Solimoes Basin (northwestern Amazonia) but were much shorter in duration, similar to 0.2 My (18.0 to 17.8 Ma) and similar to 0.4 My (14.1 to 13.7 Ma), respectively. Our results indicate that shallow marine waters covered the region at least twice during the Miocene, but the events were shortlived, rather than a continuous full- marine occupancy of Amazonian landscape over millions of years.