Derek Buitenhuis (4 April 1931 – 27 March 2014), known professionally as Derek Martinus, was an English television and theatre director. Originally an actor, he directed episodes of Z-Cars and Doctor Who, for which he was best known. He also had a long career directing stage productions.

Martinus also directed the Blake's 7 episodes "Trial" and "The Keeper" (both 1979) and over 50 episodes of Z-Cars. He worked on classic serials too, What Maisie Knew (1968), The Black Tulip (1970), A Little Princess (1973) and A Legacy (1975), plus the dramatisation of a 1970s historical fiction best seller, Penmarric.

For ITV, he directed The Paper Lads in 1977, winner of the Pye Award for best children's drama. In addition, Martinus directed the army drama series Spearhead, and several series of the children's drama Dodger, Bonzo and the rest in 1985, which also won the Pye Award.

For Swedish television, he directed a two-hour political thriller by Jan Guillou, The Wolf. After having learnt Swedish, he directed several plays in civic theatres in Sweden, many of them translated or adapted by his wife. Among others: The Homecoming by Harold Pinter, Volpone by Ben Jonson, The Shoemaker's Holiday by Thomas Dekker, The Ragged Trousered Philanthropists (the version by Stephen Lowe) and Mad Forest by Caryl Churchill. At the Gate Theatre in London, he directed the British premieres of some rarely performed Strindberg plays, translated from the Swedish by his wife Eivor.