Kathleen Mary Fitzwilliam (1826–1894) was an English actress and singer appearing regularly on the London stage in the mid 19th century.

Her first appearance in public was on 15 March 1845 at the Hanover Square Rooms as a singer on the occasion of the first performance of an original 'Stabat Mater' composed by her brother Edward Francis Fitzwilliam.

[2] Fitzwilliam's first appearance on the London stage was on 1 December 1847 at the Lyceum Theatre where she played the title role in Peggy Green which was written expressly for her by Charles Selby.

She played Ophelia with William Macready in Hamlet and appeared as Maud in The Wife's Secret with Mr and Mrs Charles Kean.

Miss Fitzwilliam made her last appearance on the stage in August 1852 in Bon Soir, Signor Pantalon at the Adelphi Theatre, and then adopted concert singing as a profession.