What Every Woman Knows

What Every Woman Knows (1908) is a four-act play written by Scottish dramatist J.M. Barrie (1860-1937). It was first presented by impresario Charles Frohman at the Duke of York's Theatre in London on 3 September 1908. It ran for 384 performances, transferring to the Hicks Theatre between 21 December 1908 and 15 February 1909.

The play was first produced in America, also by Frohman, in 1908 at Atlantic City on 18 October 1908, transferring to Broadway, at the Empire Theatre in New York City in December 1908. The production starred Maude Adams and Richard Bennett.

Written before women's suffrage, the play posits that "every woman knows" she is the invisible power responsible for the successes of the men in her life.

The play was produced by the Cape Town Repertory Theatre Society in the Rondebosch Town Hall in October 1930 and in 1943 by Gwen ffrangçon-Davies / Marda Vanne Company starring George Vollaire, Alec Bell, Siegfried Mynhardt.