Is the Great Wall of China visible from space?
Short answer: No, not without a telescope or zoom lens.
Long answer: This myth has been around since at least 1938, long before humans had any way to test it. It is unclear who came up with the idea but it was clearly speculation and not based on any evidence. When you think about it, there's no reason for the Great Wall to be any more visible than other larger human-made objects. The Great Wall isn't particularly wide, and it's made from material that doesn't really stand out from its surroundings.
China's first astronaut, Yang Liwei, confirmed that he was unable to see the Great Wall from space.
Astronauts have taken photos of the region in which the Great Wall is visible, but this is not the same as being able to see it with their own eyes. The example below was taken by astronaut Leroy Chiao from the International Space Station in 2004. The arrows indicate parts of the wall.