Guo Shoujing (Chinese: 郭守敬, 1231–1316), or Ruosi (若思), was a Chinese astronomer, engineer, and mathematician. He lived during the Yuan Dynasty (1271–1368). He is considered one of the greatest scientists in Chinese history. Guo was a major influence in the development of science.

He improved the Grand Canal, built a human-made lake (Kunming lake), created one of the most accurate calendars in history, invented some of the most accurate clocks during his time, and made many other contributions to math, astronomy, and hydraulics (water engineering).