World Seed

The World Seed feature enables the display of the random seed used to generate a Terraria world, and allows the player to input a custom seed manually when creating a new world.

Worlds generated in 1.3.4 or later will always have a world seed. In 1.3.4, an experimental feature setting was added to allow players to see, copy, and input the world seed.

Worlds generated prior to 1.3.4 will never have a viewable seed; while they were generated with a random seed, that information was only used during world creation, and was not stored in the world file.

Worlds created after 1.4.0.1 will always be able to have their seeds edited upon world creation, regardless of the experimental feature setting.

If a seed is not entered manually during world creation, a seed is automatically generated randomly.

Some secret world seeds have been discovered as easter eggs that generate worlds with strange and unique features.

Copying seeds from created worlds
Worlds with a seed display a small seed icon in the world list, allowing the player to see that world's seed and copy it to their computer's clipboard ( only). A copied seed can then be input when creating a new world, which results in a nearly identical generated world, provided it is created with identical parameters (same size, same choice between Corruption and Crimson, and same choice of difficulty).

World seeds are formatted like this:.

Size:
 * 1, if World size is Small.
 * 2, if World size is Medium.
 * 3, if World size is Large.

Difficulty:
 * 1, if World Difficulty is Classic.
 * 2, if World Difficulty is Expert.
 * 3, if World Difficulty is Master.
 * 4, if World Difficulty is Journey.

World evil:
 * 1, if World Evil is Corruption.
 * 2, if World Evil is Crimson.

Identifier: The actual unique random seed.

This formatting can be used to automatically set all the initial settings of a world to be as desired.

Trivia

 * The seed  (case-sensitive) happens to always start with a Guide named Andrew, making it easier to get the Green Cap.