Individuals tend to present their own group (the ingroup) in a systematically more favourable way (ingroup bias). By examining socially negotiated and publicly accessible Wikipedia articles about intergroup conflicts, we investigated ingroup bias at a collective level. Specifically, we compared articles about the same intergroup conflicts (e.g., the Falklands War) in the corresponding language versions of Wikipedia (e.g., the Spanish and English Wikipedia articles about the Falklands War). Study 1 featured a content coding of translated Wikipedia articles by trained raters, which showed that articles systematically presented the ingroup in a more favourable way (e.g., Argentina in the Spanish article and the United Kingdom in the English article) and, in reverse, the outgroup as more immoral and more responsible for the conflict. These findings were replicated and extended in Study 2, which was limited to the lead sections of articles but included considerably more conflicts and many participants instead of a few trained coders. This procedure allowed for separate analyses for each conflict, which showed considerable variance in the results pattern with a stronger ingroup bias for (1) more recent conflicts and (2) conflicts in which the proportion of ingroup members among the top editors was larger. Finally, a third study ruled out that these effects were driven by translations or the raters' own nationality. Therefore, this paper is the first to demonstrate ingroup bias in Wikipedia - a finding that is of practical as well as theoretical relevance as we outline in the discussion. © 2019 The Authors. British Journal of Social Psychology published by John Wiley & Sons Ltd on behalf of British Psychological Society.