Xianyu Shu (鮮於樞, ca.1257–1302) was a poet, calligrapher, collector and connoisseur of the Yuan Dynasty. His courtesy name was Boji (伯機) and pseudonym Kunxue Shanmin (困學山民).
Xianyu was probably the closest friend of Zhao Mengfu (趙孟頫). They exchanged poems and letters, wrote postscripts to paintings and calligraphies, and discussed artworks together. Zhao composed a long poem five years after Xianyu’s death, which recollected their friendship.
Xianyu excelled in semi-cursive and cursive calligraphy. He preferred to use stiff brushes and emphasized on the strength of the wrist in writing good calligraphy. Zhao Mengfu once said, “Boji and I learned cursive calligraphy together. He surpassed me to a great extent. Even though I really tried my best, I could not catch up. It is only after he died then people started recognizing my calligraphy.” (余與伯機同學草書，伯機過余遠甚，極力追之而不能及，伯機已矣，世乃稱仆能書，所謂無佛出稱尊爾。)
