Eastercon is the common name for the annual British national science fiction convention. The convention is organised by voluntary self-organising committees, rather than a permanent entity.

Eastercon attracts 800-1200 fans of science fiction, fantasy and similar genres. Programme events usually include panels, workshops, talks, quizzes, competitions, an art show, and a Dealers room selling books and other items. Events range from hard science through writing workshops to the fun and silly. People are encouraged to take part.

The very first British science-fiction convention was held in Leeds on 3 January 1937. Similar events were held in London on 10 April 1938 and 21 May 1939, Leicester as "Midvention" over the Easter bank holiday weekend of 23–26 April 1943, and in London over the Easter weekend of 8–9 April 1944.

Eastercon sees its first official convention as being held in 1948 in London. It was held over the three-day Whitsun bank holiday at the end of May with an attendance of 50 people. The conventions were held annually over Whitsun until the 1960s, when the decision was made to move the event to the four-day Easter holiday.

The pre-1960s conventions are generally considered to have been "Eastercons" even though they were not held over Easter.

Eastercons are fan-run/not-for-profit events, with the money raised by membership, advertising, etc., spent on running the convention. It is traditional that any surplus is used for the benefit of the convention members, fandom in general or donated to charity. This may include sponsoring items at other conventions, buying equipment for use by other conventions, donating to the RNIB to get works of SF literature converted to talking books for the blind, donating to the Science Fiction Foundation to fund a variety of educational projects relating to science fiction, and funding international fan visits (often through The League of Fan Funds).

Groups of fans (typically 5-8 in number) get together to form "bid committees" and plan where they want to hold the Eastercon, who they want to be their guests of honour, what the theme of the convention will be, etc. They circulate their proposals and the winning bid is chosen by a vote among the people who attend the bid session at the Eastercon two years in advance, or one year if no bid was successful at the bid session two years out. Until the early 1990s, there were commonly several bids to hold the Eastercon, but organising this event involves a lot of resources, and now it is normal for there to be only one serious bid - Reclamation in 2022 saw the first contested bid (for 2023) in some 30 years.

The Doc Weir award is voted on and presented each year at Eastercon to an "unsung hero" of British fandom.