People with dementia are publicly sharing their experiences of living with the condition and acting collectively to produce social change. Social media could support them in doing this, but no previous studies have comprehensively analysed their use of Twitter. The aims of this study were to identify how people with dementia use Twitter and examine the illness identities they create and promote online. Tweetcatcher was used to collect 2774 tweets posted over six months by 12 account holders with dementia, across three countries. Tweets were analysed thematically. Six themes were identified through the analysis: nothing about us without us, collective action, experts by experience, living with dementia not suffering from it, community, and stories of dementia. On Twitter, people with dementia are developing a collective illness identity to further a social movement that is focused on improving the lives of people with dementia. They are also communicating their personal identities by documenting their lived experiences. Twitter is being used to convey positive, rather than negative, messages about dementia. The findings of this study also show that thematic analysis can be applied to micro texts that can combine over time to form longer narratives.