Hjalmar Emil Fredrik Söderberg was a Swedish novelist, playwright, poet and journalist. His works often deal with melancholy and lovelorn characters, and offer a rich portrayal of contemporary Stockholm through the eyes of the flaneur. Söderberg is greatly appreciated in his native country, and is sometimes considered to be the equal of August Strindberg.

Born in Stockholm, Söderberg began his literary course at the Swedish news daily Svenska Dagbladet, age 20. Six years later his first novel was released, Förvillelser (Delusions, 1895), written from the viewpoint of a young dandy aimlessly idling in the capital, recklessly squandering money and love. The somber yet reflective and insightful story would prove typical of much of Söderberg's output. Subsequent to the release of Historietter (1898), a collection of twenty short stories, his next major work – Martin Bircks Ungdom (Martin Birck's Youth, 1901) – was released. Much like Förvillelser in terms of its vivid environmental depiction and acute perception, it follows the development of a young amateur poet. Söderberg's next novel, by some considered his masterpiece, was Doktor Glas (Doctor Glas, 1905). In a frightful tale of vengeance and passion, Söderberg stays true to his detached yet emotionally poignant writing style. The love story Den allvarsamma leken (The Serious Game, 1912) was Söderberg's last conventional novel.

In his later years, Söderberg turned to journalism and theological studies. He was a fierce critic of Nazism, and wrote often on the subject in the revered Resistance paper Göteborgs Handels- och Sjöfartstidning. He died in Denmark and is buried on Vestre Kirkegård in Copenhagen.

Söderberg was married to Märta Abenius (b. 1871, d. 1932) between the years 1899–1917. They had three children: actress Dora Söderberg-Carlsten (b. 1899), Tom Söderberg (b. 1900) and Mikael Söderberg (b. 1903). From 1917 he was married in Denmark to Emelie Voss (b. 1876, d. 1957), with whom he had one child: actress Betty Søderberg (b. 1910).

Söderberg had for a number of years a stormy on / off relationship with Maria von Platen (b. 1871, d.1959), a relationship which is said to have influenced him in his writing, especially his 1906 play Gertrud, and for the character of Lydia in The Serious Game.

Hjalmar Söderberg is regarded as one of the greatest writers in Swedish literature. His works are still widely read and are frequently published in new editions. His works are translated to more than twenty languages.

A recent reissue of Paul Britten Austin's translation of Doktor Glas into English, as Doctor Glas, and with a perceptive introduction by Margaret Atwood, has meant a rise in his popularity in the Anglo-Saxon literary world.