The reliability of Wikipedia concerns the validity, verifiability, and veracity of Wikipedia and its user-generated editing model, particularly its English-language edition. It is written and edited by volunteer editors who generate online content with the editorial oversight of other volunteer editors via community-generated policies and guidelines. Wikipedia carries the general disclaimer that it can be "edited by anyone at any time" and maintains an inclusion threshold of "verifiability, not truth". This editing model is highly concentrated, as 77% of all articles are written by 1% of its editors, a majority of whom have chosen to remain anonymous. The reliability of the project has been tested statistically through comparative review, analysis of the historical patterns, and strengths and weaknesses inherent in its editing process. The online encyclopedia has been criticized for its factual reliability, principally regarding its content, presentation, and editorial processes. Studies and surveys attempting to gauge the reliability of Wikipedia have mixed results, with findings varied and inconsistent. Wikipedia's reliability was frequently criticized in the 2000s but has improved over time; it has been generally praised in the late 2010s and early 2020s.

Select assessments of its reliability have examined how quickly vandalism—content perceived by editors to constitute false or misleading information—is removed. Two years after the project was started, in 2003, an IBM study found that "vandalism is usually repaired extremely quickly—so quickly that most users will never see its effects". The inclusion of false or fabricated content has, at times, lasted for years on Wikipedia due to its volunteer editorship. Its editing model facilitates multiple systemic biases: namely, selection bias, inclusion bias, participation bias, and group-think bias. The majority of the encyclopedia is written by male editors, leading to a gender bias in coverage and the make up of the editing community has prompted concerns about racial bias, spin bias, corporate bias, and national bias, among others. An ideological bias on Wikipedia has also been identified on both conscious and subconscious levels. A series of studies from Harvard Business School in 2012 and 2014 found Wikipedia "significantly more biased" than Encyclopædia Britannica but attributed the finding more to the length of the online encyclopedia as opposed to slanted editing.

The prevalence of non-neutral or conflict-of-interest editing and the use of Wikipedia for "revenge editing" has attracted publicity for inserting false, biased, or defamatory content into articles, especially biographies of living people. Articles on less technical subjects, such as the social sciences, humanities, and culture, have been known to deal with misinformation cycles, cognitive biases, coverage discrepancies, and editor disputes. The online encyclopedia does not guarantee the validity of its information. It is seen as a valuable "starting point" for researchers when they pass over content to examine the listed references, citations, and sources. Academics suggest reviewing reliable sources when assessing the quality of articles.

Its coverage of medical and scientific articles such as pathology, toxicology, oncology, pharmaceuticals, and psychiatry were compared to professional and peer-reviewed sources in a 2005 Nature study. A year later Encyclopædia Britannica disputed the Nature study, whose authors, in turn, replied with a further rebuttal. Concerns regarding readability and the overuse of technical language were raised in studies published by the American Society of Clinical Oncology (2011), Psychological Medicine (2012), and European Journal of Gastroenterology and Hepatology (2014). Wikipedia's popularity, mass readership, and free accessibility has led the encyclopedia to command a substantial second-hand cognitive authority across the world.