Cài Lún (Wade-Giles: Ts'ai Lun, 蔡倫) (c. AD 50—121), courtesy name Jingzhong (敬仲), was a Chinese eunuch, who is conventionally regarded as the inventor of paper.

He was born in Guiyang during the Eastern Han Dynasty, and became a paperwork secretary (中常侍) of Emperor Hedi. For papermaking, he tried materials like bark, hemp, silk, and even fishing net, but his exact formula has been lost to history. The emperor was pleased with the invention and granted Cai an aristocratic title and great wealth. Later, he became involved in intrigue and was imprisoned. There, he committed suicide by drinking poison after taking a bath and dressing in fine robes.

While paper is widely used worldwide today, the creator of this extremely important invention is little-known outside East Asia. Cai invented paper in AD 105. It immediately became widely used in China. In 751, some Chinese paper makers were captured by Arabs after Tang troops were annihilated in the Battle of Talas River. The techniques of paper making then spread to the West.

Cai's contribution is considered one of the most important inventions in history, since it enabled China to develop its civilization much faster than with earlier writing materials (primarily bamboo), and it did the same with Europe when it was introduced in the 12th century or the 13th century.