Frederick Lonsdale (5 February 1881 – 4 April 1954) was a British playwright known for his librettos to several successful musicals early in the 20th century, including King of Cadonia (1908), The Balkan Princess (1910), Betty (1915), The Maid of the Mountains (1917), Monsieur Beaucaire (1919) and Madame Pompadour (1923).

Lonsdale's more substantial than usual dialogue for the show's Ruritanian comic opera plot won King of Cadonia fine notices and helped the musical to a long career.

His next success was also for Curzon, The Balkan Princess (1910); this was little more than King of Cadonia with the sexes reversed, but it enjoyed a good London run, a long and wide provincial tour, and foreign productions.

He adapted Booth Tarkington's Monsieur Beaucaire (1919, with music by André Messager) as a highly successful light opera and Jean Gilbert's Die Frau im Hermelin (1922, The Lady of the Rose) and Katja, die Tänzerin (1925), as well as Leo Fall's Madame Pompadour (1923).

He also wrote the successful original book to the Parisian tale of The Street Singer (based on a 1912 film of the same name for Phyllis Dare (1924) and Lady Mary (1928).