Precipitation in most of the Amazon shows multi-decadal fluctuations that were linked to oceanic forcing in the Atlantic. This modeling study shows that vegetation dynamics may play a major role in such low-frequency variability in the Amazon. Despite the large amount of annual precipitation, the presence of a dry season (albeit short) facilitates a strong impact of dynamic vegetation on precipitation persistence in the model. The year-to-year variation of net primary productivity (NPP) is dominated by that of the dry season NPP. As a result, above-normal (below-normal) precipitation in a particular year can enhance (suppress) vegetation growth, leading to widespread increase (decrease) of vegetation density in the subsequent year. Precipitation in the subsequent year is therefore more likely to be above (below) normal. This damping effect of vegetation enhances low-frequency variability of precipitation and leads to recurrent droughts or floods, a result previously considered characteristic of arid and semi-arid regions.