Earliest Paleocene megafloras from North America are hypothesized to be low diversity and dominated by long-lived cosmopolitan species following the Cretaceous-Paleogene (K/Pg) mass extinction. However, megafloras used to develop this hypothesis are from the Northern Great Plains of North America, and relatively little is known about floras from southern basins. Here, we present a quantitative analysis of an earliest Paleocene megaflora (<350 kyr after K/Pg boundary) from the Ojo Alamo Sandstone in the San Juan Basin (SJB), New Mexico. The megaflora, comprised of 53 morphotypes, was dominated by angiosperms, with accessory taxa composed of pteridophytes, lycophytes, and conifers. Diversity analyses indicate a species rich, highly uneven, and laterally heterogeneous flora. Paleoclimate estimates using multivariate and univariate methods indicate warm temperatures and relatively high precipitation consistent with a modern tropical seasonal forest. When compared to contemporaneous floras from the Denver Basin (DB) of Colorado and the Williston Basin (WB) of North Dakota, the SJB flora had significantly higher species richness but lower evenness. Paleoclimate estimates from the SJB were 7-14 °C warmer than the DB and WB, indicating a shift from a temperate forest in the Northern Great Plains to a tropical forest in the SJB. These results demonstrate the presence of a latitudinal floral diversity and paleoclimatic gradient during the earliest Paleocene in western North America. We hypothesize that the warm, wet conditions in the earliest Paleocene SJB drove rapid rates of speciation following the K/Pg boundary resulting in a diverse and heterogenous flora.