How effective is fact-checking in countervailing “alternative facts,” i.e., misleading statements by politicians? In a randomized online experiment during the 2017 French presidential election campaign, we subjected subgroups of 2480 French voters to alternative facts by the extreme-right candidate, Marine Le Pen, and/or corresponding facts about the European refugee crisis from official sources. This is the dataset from this experiment. We also include the replication code for the article: Barrera, Oscar, Sergei Guriev, Emeric Henry, and Ekaterina Zhuravskaya. 2020. "Facts, alternative facts, and fact checking in times of post-truth politics." Journal of Public Economics, 182: 104123. DOI: https://doi.org/10.1016/j.jpubeco.2019.104123