X-rays were discovered by Wilhelm Conrad Röntgen, a German physicist, on November 8, 1895.

He was experimenting with vacuum tubes and noticed that a fluorescent screen nearby was glowing when the tube was operating, even though it was covered with black cardboard. He deduced that some invisible rays were causing the effect, and he called them "X-rays" because their nature was unknown.

Röntgen's discovery revolutionized medical diagnostics and earned him the first Nobel Prize in Physics in 1901.

Source: The Nobel Prize in Physics 1901