Cai Yong (132/133[1] – c. June 192[2]), courtesy name Bojie, was a Chinese astronomer, calligrapher, historian, mathematician, musician, politician, and writer of the Eastern Han dynasty.

After that, Cai Yong studied composition, mathematics, astronomy, pitch-pipes and music under Hu Guang (胡廣), one of the highest-ranking officials in the Han imperial court.

Ten years later in the early 170s, Cai Yong served as a clerk under the official Qiao Xuan, who greatly admired his abilities.

Emperor Ling (r. 168–189) approved, and the result was the Xiping Stone Classics completed in 183, which set the canon for future generations of scholars.

Nine months later, he cited to the throne that his work on the dynastic history and classics were at risk from enemy raids, and was allowed back to the capital.

When the warlord Dong Zhuo came to power in 189 and controlled the central government, he summoned Cai Yong back to the imperial capital Luoyang.

Cai Yong and other government officials pleaded with Wang Yun to allow him to finish his work on the history of Han, but Wang Yun denied them, saying: "In ancient times, Emperor Wu failed to kill Sima Qian, and so allowed him to write a book of slander which was passed down to later times.