Otto Julius Bierbaum (28 June 1865 – 1 February 1910) was a German writer.

Bierbaum was born in Grünberg, Silesia. After studying in Leipzig, he became a journalist and editor for the journals Die freie Bühne, Pan and Die Insel. His literary work was varied. As a poet he used forms like the Minnesang or the folksong and the Anacreontics style. Composers such as Pauline Volkstein set his texts to music.

In 1897 Bierbaum published his novel Stilpe which inspired Ernst von Wolzogen to establish, in 1901, the first cabaret venue ever in Berlin, the Überbrettl . His novel Zäpfel Kerns Abenteuer was an adaptation of Carlo Collodi's Pinocchio. Bierbaum's final novel, Yankeedoodlefahrt, was published in 1909 and is the source of the winged words "Humor ist, wenn man trotzdem lacht" ("Humor is when you laugh anyway"), which has become a proverb in modern German.

Bierbaum died at Kötzschenbroda near Dresden.