Why do some voters believe that there is electoral fraud when this belief contradicts the best available evidence? While the literature on public opinion has explored misperceptions’ pervasiveness and the factors that contribute to them in advanced industrial democracies, the present study analyzes motivated reasoning among voters in a young democracy, Mexico. This study highlights the important role that partisanship plays in voters’ likelihood to believe the allegations of electoral fraud in the 2006 presidential election in Mexico, which continues polarizing both political elites and the electorate even today. To understand the mechanisms at work, this research finds that it is not voters lacking information but rather voters with high levels of affective polarization and conspiratorial thinking who are more likely to believe that there was electoral fraud. The study also includes a survey experiment that fact-checked the belief in the alleged electoral fraud. Consistent with motivated reasoning theory, MORENA partisans resisted efforts to reduce their misperception. The findings of this study contribute to our understanding of the conditions that make some voters hold misperceptions in young democracies.