Hjalmar Eilif Emanuel Peterssen (4 September 1852 – 29 December 1928) was a Norwegian painter. He is most commonly associated with his landscapes and portraits. He gained early recognition for the history painting Christian II signing the Death Warrant of Torben Oxe and established himself as one of Norway's foremost portrait painters, with portraits of, among others, Henrik Ibsen and Edvard Grieg. He also became known for his landscape paintings, and became part of the artist circle known as the Skagen Painters. He also became known for his design in 1905 of Norway's national coat of arms with the Norwegian lion, which was used by the government and the royal house. The design is still used in the royal coat of arms and the royal flag.

Hjalmar Eilif Emanuel Peterssen was born in Christiania, now Oslo, Norway. He was the son of Jon Peterssen (1814–1880) and Anne Marie Andersen (1812–1887). He grew up in the neighborhood of Hegdehaugen in the district of Frogner.

He attended the Norwegian National Academy of Craft and Art Industry (Statens håndverks- og kunstindustriskole) in Christiania from 1866–1870. He entered the Johan Fredrik Eckersberg School of Painting in 1869. He trained with Knud Bergslien and Morten Müller in the autumn of 1870. In 1871 he left Oslo to study at the Royal Danish Academy of Fine Arts in Copenhagen. Later that year he moved to Karlsruhe, where he was a student of Ludwig des Coudres at the Academy of Fine Arts, Karlsruhe and Wilhelm Riefstahl at Weimar Saxon-Grand Ducal Art School. In the fall of 1873 Peterssen traveled to Munich where he studied at the Academy of Fine Arts, Munich under Wilhelm von Diez and Franz von Lenbach. In Munich Peterssen met other famous artists such as Arnold Böcklin and Karl von Piloty.

Peterssen made several trips to France and Italy. In 1896 he went to Arques-la-Bataille in Normandy, where he painted several landscapes, and from France he went together with his family to Rome in 1897. In 1903, Peterssen again visited Italy and in Rapallo near Genova, he painted the impressionist motif Winter in the South (Washerwomen in Rapallo). During the dissolution of Union between Sweden and Norway in 1905, Peterssen was commissioned to design the new coat of arms of Norway.

In his later years Peterssen travelled all over Norway to paint landscapes. He made several visits to Skogstad in Valdres, where he was particularly inspired by the great mountain landscape. In 1920–21, he made his last travel abroad to Cagnes-sur-Mer and Saint-Paul-de-Vence in Provence where he painted several landscapes of the small villages on the hills between Nice and Cannes.

Peterssen made his breakthrough as a painter in Munich with the history painting Christian II signing the Death Warrant of Torben Oxe (1876) which was acquired by the Verbindung für historische Kunst in Stuttgart. In Munich, he painted one of his biggest paintings, the altarpiece The Crucifixion (disappeared) for Johannes Church (Johanneskirken) in Oslo which was demolished in 1928. Later he was to paint nine more altarpieces and a church decoration, The Ascension (1908–1909) in Ullern Church in Oslo.

Peterssen is also known for his portraits. In Munich he painted some of his best portraits, of artist friends such as Harriet Backer and Hans Heyerdahl and of the German painters Anton Windmaier and Adolf Heinrich Lier. He painted Princess Anna Elisabeth Reuss at Schloss Schleiz in Gera during 1878.

Peterssen was influenced by the brownish palette of the Munich School painters. However Peterssen was soon to adopt the increasingly popular En plein air style when he traveled to Italy in 1879. He visited Sora in 1880 together with the Danish painter Peder Severin Krøyer, and in this mountain village he painted his great naturalistic work Siesta in an osteria in Sora. A sharp realism is characteristic of his big canvas Piazza Montanara (1883) painted in Rome.