Théo De Joncker (11 April 1894, in Brussels – 12 July 1964, in Asse) was a Belgian composer.
Théo De Joncker was a student of August de Boeck and Paul Gilson.
For years he conducted the orchestra of the Belgian National Radio (NIR), but he also enjoyed a conducting career outside of Belgium.
As a composer, he belonged to the Brussels group Les Synthétistes.
This group was found by students of Paul Gilson as a way to celebrate their teacher's 60th birthday and included Francis de Bourguignon, Théo De Joncker, Marcel Poot, Maurice Schoemaker, Jules Strens, René Bernier, and Robert Otlet.