(William) Denis Johnston (18 June 1901 – 8 August 1984) was an Irish writer. Born in Dublin, he wrote mostly plays, but also works of literary criticism, a book-length biographical essay of Jonathan Swift, a memoir and an eccentric work on cosmology and philosophy. He also worked as a war correspondent, and as both a radio and television producer for the BBC. His first play, The Old Lady Says "No!", helped establish the worldwide reputation of the Dublin Gate Theatre; his second, The Moon in the Yellow River, has been performed around the globe in numerous productions featuring such storied names as James Mason, Jack Hawkins, Claude Rains, Barry Fitzgerald, James Coco and Errol Flynn. Later plays dealt with the life of Swift, the Easter Rising of 1916,, the pursuit of justice, and the fear of death. He wrote two opera libretti and a pageant.

Johnston was the only child of William John Johnston from Magherafelt, a barrister (later an Irish Supreme Court judge), and his wife, Kathleen (née King), a teacher and singer from Belfast. They were Presbyterians and liberal home rulers. Johnston was to see the family home in Dublin occupied by rebels during the 1916 Easter Rising.

Johnston was educated at St Andrew's College, Dublin (1908–15, 1917–19), and Merchiston Castle School, Edinburgh (1915–16). In 1918, he attempted to join Sinn Féin, offering to supply the party with weapons taken from his Officer Training Corps. In 1922, while reading history and law at Christ's College, Cambridge (1919–23) he tried to enlist in the civil-war Free State army. His time at Cambridge was a success: in addition to graduating with MA (Cantab.) and LLM degrees, he was also elected President of the Cambridge Union in 1921. He went on to study at the Harvard Law School (1923–4) and entered King's Inns (Dublin) and the Inner Temple (London).

In London, developing his interest in the theatre, Johnston abandoned plans for a legal and political career.

Johnston was a protégé of Yeats and Shaw, and had a stormy friendship with Seán O'Casey. He was a pioneer of television and war reporting. He worked as a lawyer in the 1920s and '30s before joining the BBC as a writer and producer, first in radio and then in the fledgling television service. His broadcast dramatic work included both original plays and adaptations of the work of many different writers.

"Passionate in his radical scepticism and loathing of what he saw as the pernicious influence of the Roman Catholic Church", at the end of 1933, Johnston joined the trade unionist John Swift, the Dublin novelist Mary Manning, and fellow northerner, the socialist Jack White, in forming The Secular Society of Ireland. "Convinced that clerical domination in the community is harmful to advance", the society sought "to establish in this country complete freedom of thought, speech and publication, liberty for mind, in the widest toleration compatible with orderly progress and rational conduct". Among other things it aimed to terminate ”the clerically-dictated ban on divorce”, “the Censorship of Publications Act” and “the system of clerical management, and consequent sectarian teaching, in schools.”

This was at a time of heightened clerical militancy and as soon the meeting place of the Society (from which it distributed the British journal The Freethinker) was exposed, it had to shift to private houses outside of Dublin. In 1936 Johnson and the other members wound the society up and donated the proceeds to the government of the beleaguered Spanish Republic. Johnston had become a recognised man of the left: in 1930 he had joined the Irish Friends of Soviet Russia, and though never a party member, until as late as the 1950s he professed faith in a communist future.

During the Second World War he served as a BBC war correspondent, reporting from El Alamein, through the Italian campaign, to Buchenwald and Hitler's Berghof. For this he was awarded an OBE, a Mention in Despatches, and the Yugoslav Partisans Medal. He then became Director of Programmes for the television service.