Ernst Wolf Alexander Oskar Harnack (15 July 1888 – 5 March 1945), granted the title von Harnack in 1914, was an official of the Prussian provincial government, a German politician, and a German Resistance fighter. He was arrested, tried and executed in March 1945 at Plötzensee Prison for political opposition to the Nazi Party.
Harnack was born in Marburg as the son of the theologian Adolf von Harnack (1851–1930) and Amalie Thiersch (1858–1937), the granddaughter of the chemist Justus von Liebig. On 25 March 1916, in Hindenburg (Upper Silesia), he married Anna (Änne) Wiggert (b. 5 October 1894, in Göttelborn, Ottweiler district, Saarland; d. 22 August 1960, in Berlin), daughter of the Prussian official Ernst Wiggert and Elizabeth Schmidt. They had two sons and three daughters.
After a year's private instruction, Harnack attended the Joachimsthalsche Gymnasium in Berlin, where he took the Abitur in 1907. Then he studied law at the University of Marburg and later in Berlin. On 6 May 1911, he took the bar examination and began afterwards working at a magistrate's Court in the Lichterfelde of Berlin. From 1 October 1911 to 30 September 1912, he spent a year doing military service as an Einjährig-Freiwilliger. From 2 August 1914 to 15 May 1915 he served in the First World War and was active with civil administration in Congress Poland. He began his official career on 8 March 1913 as a governmental junior lawyer in Oppeln. On 29 June 1918, he was appointed as a civil servant (Regierungsassessor) in the Ministry of Science, Art and National Education. He was promoted on 24 January 1921 to Regierungsrat. From 15 August 1921 to 9 November 1923 and from 1 June 1924 to 31 May 1925 he was a Landrat in Hersfeld-Rotenburg, with an appointment as Landrat in Uecker-Randow in the period between. On 1 June 1925 he was appointed vice head of the regional government (Regierungsvizepräsident) in Hanover. He was transferred on 1 April 1927 in the same position in Cologne. On 8 August 1929, he started as of the head of the provincial government (Regierungspräsident) in Merseburg.