Sebastian David Fitzek is a German writer and journalist. His first book, Therapy, was a bestseller in Germany in 2006, toppling The Da Vinci Code from the first position. Fitzek is currently one of the most successful writers of Germany.

Therapy is a psychological thriller and the debut novel by novelist Sebastian Fitzek published in 2006. The novel has already sold eight million copies worldwide. In addition, the novel held the number one spot on the German magazine Spiegel's bestseller list for several weeks. In 2023 it was adapted into a TV series titled Sebastian Fitzek's Therapy.