| Loncon I, the 15th World Science Fiction Convention | |
| Venue: | King's Court Hotel |
| Dates: | 6–9 September 1957 |
| Attendance: | 268 |
The 15th World Science Fiction Convention (Worldcon), also known as Loncon I, was held on 6–9 September 1957 at the King's Court Hotel in London, United Kingdom. It was the first Worldcon held outside North America.
The chairman was Ted Carnell.
Attendance was 268.
Events included a "fancy dress ball" on the evening of Friday, 6 September.
The winners were:
Because the 1957 International Fantasy Award was being given out in London at the same time, Loncon I chose not to compete with this similar literary award, which was given to J.R.R. Tolkien's The Lord of the Rings. As a result, only the three Hugo Awards for Best Professional Magazine and Best Fanzine were given out at the 1957 Worldcon. Both the International Fantasy Award and the Hugo Award shared the very same Chesley Bonestell-influenced finned rocketship design that year, the only time this has happened.