RationalWiki is an online wiki which is written from a scientific skeptic, secular, and progressive perspective. Its stated goals are to "analyze and refute pseudoscience and the anti-science movement, document crank ideas, explore conspiracy theories, authoritarianism, and fundamentalism, and analyze how these subjects are handled in the media." It was created in 2007 as a counterpoint to Conservapedia, a Christian fundamentalist website, after an incident in which some editors of Conservapedia were banned. RationalWiki has been described as liberal in contrast to Conservapedia.

In April 2007, Peter Lipson, a doctor of internal medicine, attempted to edit Conservapedia's article on breast cancer to include evidence against Conservapedia's claim that abortion was linked to the disease. Conservapedia is an encyclopedia established by Andy Schlafly as an alternative to Wikipedia, which Schlafly perceived as suffering from a liberal, atheist, and "anti-American" bias. He and Conservapedia administrators "questioned [Lipson's] credentials and shut down debate". After being reverted and blocked, "Lipson and several other contributors quit trying to moderate the articles [on Conservapedia] and instead started their own website, RationalWiki".

Prior to 2010, RationalWiki's domains were registered to Trent Toulouse, and the wiki was hosted from a server located in his home. In 2010, Trent Toulouse incorporated a nonprofit organization, the RationalWiki Foundation Inc., to manage the affairs and pay the operational expenses of the website. In July 2013, the RationalWiki Foundation changed its name to the RationalMedia Foundation, stating that its aims extended beyond the RationalWiki site alone.

In April 2025, the RationalMedia Foundation was sued for defamation by various plaintiffs who were covered on RationalWiki. In the same month, despite concerns about the foundation dissolving, it reached an out of court settlement with the plaintiffs where it agreed to delete their articles.

RationalWiki aims to provide information about pseudoscientific theories and to educate "individuals with unorthodox views".

RationalWiki differs in several ways from the philosophy of Wikipedia and some other informational wikis. It is written from a self-described "snarky point of view" and "scientific point of view" (both abbreviated as SPOV) rather than a "neutral point of view" (NPOV), and publishes opinion, speculation, and original research. Many RationalWiki articles satirically describe beliefs that RationalWiki opposes, especially when covering topics such as alternative medicine or fundamentalist Christians.

Some activity on RationalWiki was used for critiquing and "monitor[ing] Conservapedia". RationalWiki contributors, some of whom are former Conservapedia contributors, are often highly critical of Conservapedia. According to a 2007 Los Angeles Times article, RationalWiki members "by their own admission" vandalize Conservapedia. Lester Haines of The Register stated: "Its entry entitled 'Conservapedia:Delusions' promptly mocks the claims that 'Homosexuality is a mental disorder', 'Atheists are sociopaths', and 'During the 6 days of creation G-d placed the Earth inside a black hole to slow down time so the light from distant stars had time to reach us'."

Both Yan et al. 2019 and Knoche et al., two articles about classifying a writer's biases via text analysis, asserted that Conservapedia was "conservative" and RationalWiki was "liberal". Mic described RationalWiki as "progressive".