Desmond Patrick Costello (31 January 1912 – 23 February 1964) was a New Zealand linguist, soldier, diplomat, university professor who was accused of being an agent for the Soviet GRU and of having sparked the first in a series of spy scandals in the 1960s which brought down the Macmillan government in the UK.
He was born in Auckland, New Zealand.
Eminent New Zealand author Sir James McNeish researched and wrote a biography of Paddy Costello entitled The Sixth Man: the extraordinary life of Paddy Costello (Random House, 2007). In the preface to this book, Philip Knightley writes that "This long overdue biography sets the record straight. It convincingly shows that the slim evidence for the traitor charge was invented, exaggerated and misinterpreted." Costello's MI5 file was released to the public in April 2017, but it fails to settle the issue of his supposed treachery.
Paddy Costello also features in McNeish's multiple biography Dance of the Peacocks: New Zealanders in exile in the time of Hitler and Mao Tse Tung (Vintage, 2003) about five exceptional New Zealanders, four of them Rhodes scholars, who became caught up in the turmoil of their time. Costello was the sixth man.