With improvements in taxonomic resolution in recent years, the Amazon basin is increasingly recognized as containing restricted-range taxa and areas of endemism. For many of these taxa, rivers delimit their geographic distributions and separate sister species. Among most lizards the geographic pattern is not clear. We attempt to determine the existence of cryptic diversity in the Spectacled Lizard Leposoma osvaldoi by analyzing the geographic variation in its morphology. We specifically tested whether the Rio Madeira, Rio Purus, Rio Aripuanã, and Rio Roosevelt delimit differentiated lizard forms. Using multivariate analysis, we detected that males of L. osvaldoi have larger heads than females but females have longer bodies. Large rivers, such as the Rio Purus and Rio Madeira, do not play a significant role in the geographic variation of L. osvaldoi. However, specimens from two localities on opposite banks of the upper Rio Aripuanã occupy the lower and upper ranges of variation in L. osvaldoi and are distinct from all other specimens analyzed. This unusual pattern differs from one of the main river hypothesis predictions, which is that similarity between individuals on opposite river banks should increase with decreasing barrier strength. We hypothesize that the differences in river dynamics between the lower and upper Rio Aripuanã during mid-Miocene through the Pleistocene could explain our results, although phylogeographic evidence is lacking to support such a hypothesis. The elucidation of recent and ancient processes that shaped diversity in Leposoma could provide important clues to understand the formation of the megadiversity of Amazonia.