Fibromyalgia is a multi-factorial illness primarily characterised by widespread chronic pain and fatigue, with several symptoms and associated conditions. Due to a lack of clinical awareness and an absence of objective diagnostic measures, fibromyalgia patients often engage with online health information. The aim is to investigate the completeness and trustworthiness of the information available online on fibromyalgia. Google.co.uk was searched for 'fibromyalgia', the first 200 webpages were imported and 148 were analysed for standard health information quality criteria (JAMA score, HONcode) as well as completeness of information in terms of symptoms, causes and treatments mentioned. The most frequent typology of webpages was from health professionals (38%), with commercial websites being less frequent (7%). Overall, the quality, completeness and accessibility of online health information was poor. Completeness of coverage for symptoms, causes and associated conditions was especially lacking, with pages from not-for-profit organisations discussing the highest number of symptoms (median 8, min 0, max 11, interquartile range, IQR 4.5; n = 14) compared to the rest of the websites in the search engine results (median 4, min 0, max 11, IQR 4; n = 134). Mean readability was grade 9 (median 9, min 1, max 18, IQR 3), with only 8% websites meeting the recommended readability of grade 6. The Internet provides incomplete information on fibromyalgia, which does not fulfil the most queried aspect(s) by patients, symptoms, and may be difficult to understand by lay persons. Not-for-profit organisations provide the most complete information compared to other types of websites.