Fritz Jakob Haber (German: [ˈfʁɪt͡s ˈhaːbɐ] ^ⓘ; 9 December 1868 – 29 January 1934) was a German chemist who received the Nobel Prize in Chemistry in 1918 for his invention of the Haber process, a method used in industry to synthesize ammonia from nitrogen gas and hydrogen gas. This invention is important for the large-scale synthesis of fertilizers and explosives. It is estimated that a third of annual global food production uses ammonia from the Haber–Bosch process, and that this food supports nearly half the world's population. For this work, Haber has been called one of the most important scientists and industrial chemists in human history. Haber also, along with Max Born, proposed the Born–Haber cycle as a method for evaluating the lattice energy of an ionic solid.

Haber, a known German nationalist, is also considered the "father of chemical warfare" for his years of pioneering work developing and weaponizing chlorine and other poisonous gases during World War I. He first proposed the use of the heavier-than-air chlorine gas as a weapon to break the trench deadlock during the Second Battle of Ypres. His work was later used, without his direct involvement, to develop the Zyklon B pesticide used for the killing of more than 1 million Jews in gas chambers in the greater context of the Holocaust.

After the Nazis' rise to power in 1933, Haber resigned from his position. Already in poor health, he spent time in various countries before Chaim Weizmann invited him to become the director of the Sieff Research Institute (now the Weizmann Institute) in Rehovot, Mandatory Palestine. He accepted the offer but died of heart failure mid-journey in a Basel, Switzerland hotel on 29 January 1934, aged 65.

Haber was born in Breslau, Kingdom of Prussia (now Wrocław, Poland), into a well-off Jewish family. Though Haber was a common family name in Breslau, the family has been traced back to a great-grandfather, Pinkus Selig Haber, who was a wool dealer from Kempen (now Kępno, Poland). An important Prussian edict of 13 March 1812 determined that Jews and their families, including Pinkus Haber, were "to be treated as local citizens and citizens of Prussia". Under such regulations, members of the Haber family were able to establish themselves in respected positions in business, politics, and law.

Haber was the son of Siegfried and Paula Haber, who were first cousins who married in spite of considerable opposition from their families. Haber's father Siegfried was a well-known merchant in the town, who had founded his own business in dye pigments, paints and pharmaceuticals. Paula experienced a difficult pregnancy and died three weeks after Fritz's birth, leaving Siegfried devastated and Fritz in the care of various aunts. When Haber was about six years old, Siegfried remarried to Hedwig Hamburger. Siegfried and his second wife had three daughters: Else, Helene, and Frieda. Although his relationship with his father was distant and often difficult owing to Fritz being associated with the death of his first wife, Haber developed close relationships with his stepmother and his half-sisters. Siegfried displayed love and care for his three daughters but never fully accepted Fritz as his son.

By the time Fritz was born, the Habers had to some extent become assimilated into German society. He attended primary school at the Johanneum School, a "simultaneous school" open equally to Catholic, Protestant, and Jewish students. At age 11, he went to school at the St Elizabeth [de] classical school in Breslau, in a class evenly divided between Protestant and Jewish students. His family supported the Jewish community and continued to observe many Jewish traditions, but were not strongly associated with the synagogue. Haber identified strongly as German, less so as Jewish.

Haber successfully passed his examinations at the St Elizabeth gymnasium in September 1886. Although his father wished him to become an apprentice in the dye company, Haber obtained his father's permission to study chemistry at the Friedrich Wilhelm University in Berlin (today the Humboldt University of Berlin), with the director of the Institute for Chemistry A. W. Hofmann. Haber was disappointed by his initial winter semester (1886–87) in Berlin, and arranged to attend Heidelberg University for the summer semester of 1887, where he studied under Robert Bunsen. He then returned to Berlin, to the Technische Hochschule in Charlottenburg (today Technische Universität Berlin).

In the summer of 1889, Haber was conscripted and left university to perform his one-year volunteer service in the Sixth Field Artillery Regiment. Upon its completion, he returned to Charlottenburg where he became a student of Carl Liebermann. In addition to Liebermann's lectures on organic chemistry, Haber also attended lectures by Otto Witt on the chemical technology of dyes.