While the language of the internet has been an increasingly popular research topic, there remain many understudied areas and topics which deserve more attention. This study explores register variation within the social media website Reddit using the multi-dimensional approach developed by Douglas Biber. Reddit, the third most popular English-language social media website after the giants Facebook and Twitter, is made up of thousands of user-created ‘subreddits’, subcommunities centered around different topics, where users make posts and comment on them. Many different communities and topic areas under one roof makes Reddit a particularly fruitful source of research material. In this paper, three register dimensions are extracted from data collected over one month from a group of thirty-seven subreddits: ‘On-line Subjective Production’, ‘Informational Style’ and ‘Instructional Focus’. These dimensions describe register variation within Reddit in meaningful ways. They are also in line with suggested register universals (Biber 2014).