The Neogene was a critical interval in the establishment of the modern geography and biotic composition of Amazonia. Because the region is covered with rainforest vegetation and lacks extensive outcrops, most of the understanding of its geological and evolutionary history relies on the study of rock cores. We studied the Miocene paleoenvironments and paleoclimate of borehole 1-AS-15-AM, drilled in western Amazonia, by analyzing the palynological content of 107 samples from the Solimões Formation. Graphic correlation indicates that the borehole spans palynological zones T13 to T16 (middle to late Miocene). We found 342 morphospecies of pollen and spores, including 280 angiosperm taxa and 62 pteridophyte taxa. Sixteen new species are described: 12 angiosperms and four pteridophytes. Pollen counts are dominated by elements associated with aquatic grasses, palms and ferns that can be associated with the Pebas ecosystem. We also found 11 morphospecies of dinoflagellate cysts and microforaminiferal linings between 115.4 m and 118.95 m that correlate with the middle Miocene marine flooding event of western Amazonia. Average Miocene temperature was estimated to be 24.9 °C (min = 14.40, max = 27.30) while precipitation was 1946 mm/y (min = 1,080, max = 3000), not significantly different from Holocene estimates.