Charles Francis Richter (/ˈrɪktər/; April 26, 1900 – September 30, 1985) was an American seismologist and physicist. He is the namesake and one of the creators of the Richter scale, which, until the development of the moment magnitude scale in 1979, was widely used to quantify the size of earthquakes. Inspired by Kiyoo Wadati's 1928 paper on shallow and deep earthquakes, Richter first used the scale in 1935 after developing it in collaboration with Beno Gutenberg; both worked at the California Institute of Technology.

Richter was born in Overpeck, Ohio. Richter had German heritage: his great-grandfather was a Forty-Eighter, coming from Baden-Baden (today in Baden-Württemberg, Germany) in 1848 in the wake of the German revolutions of 1848–1849. Richter's parents Frederick William and Lillian Anna (Kinsinger) Richter, were divorced when he was very young. He grew up with his maternal grandfather, who moved the family (including his mother) to Los Angeles in 1909. After graduating from Los Angeles High School he attended Stanford University and received his undergraduate degree in 1920. In 1928, he began work on his PhD in theoretical physics from the California Institute of Technology, but, before he finished it, he was offered a position at the Carnegie Institute of Washington. At this point, he became fascinated with seismology (the study of earthquakes and the waves they produce in the earth). Thereafter, he worked at the new Seismological Laboratory in Pasadena, under the direction of Beno Gutenberg. In 1932, Richter and Gutenberg developed a standard scale to measure the relative sizes of earthquake sources, called the Richter scale. In 1937, he returned to the California Institute of Technology, where he spent the rest of his career, eventually becoming professor of seismology in 1952.

Richter went to work at the Carnegie Institution of Washington in 1927 after Robert Andrews Millikan offered him a position as a research assistant there, where he began a collaboration with Beno Gutenberg. The Seismology Lab at the California Institute of Technology wanted to begin publishing regular reports on earthquakes in southern California and had a pressing need for a system of measuring the strength of earthquakes for these reports. Together, Richter and Gutenberg devised the scale that would become known as the Richter scale to fill this need, based on measuring quantitatively the displacement of the earth by seismic waves, as Kiyoo Wadati had suggested.

The pair designed a seismograph that measured this displacement and developed a logarithmic scale to measure intensity. The name "magnitude" for this measurement came from Richter's childhood interest in astronomy – astronomers measure the intensity of stars in magnitudes. Gutenberg's contribution was substantial, but his aversion to interviews contributed to his name being left off the scale. After the publication of the proposed scale in 1935, seismologists quickly adopted it for use in measuring the intensity of earthquakes.

Richter remained at the Carnegie Institution until 1936, when he obtained a post at the California Institute of Technology, where Beno Gutenberg worked. Gutenberg and Richter published Seismicity of the Earth in 1941. Its revised edition, published in 1954, is considered a standard reference in the field.

Richter became a full professor at the California Institute of Technology in 1952. In 1958, he published Elementary Seismology based on his undergraduate teaching notes. As Richter seldom published in peer reviewed scientific journals, that is often considered his most important contribution to seismology. Richter spent 1959 and 1960 in Japan as a Fulbright scholar. Around this time in his career, he became involved in earthquake engineering through development of building codes for earthquake prone areas. The city government of Los Angeles removed many ornaments and cornices from municipal buildings in the 1960s as a result of Richter's awareness campaigns.

After the 1971 San Fernando earthquake, the city cited Richter's warnings as important in preventing many deaths. Richter had retired in 1970.

At the time when Richter began a collaboration with Gutenberg, the only way to rate shocks was a scale developed in 1902 by the Italian priest and geologist Giuseppe Mercalli. The Mercalli scale uses Roman numerals and classifies earthquakes from I to XII, depending on how buildings and people responded to the tremor. A shock that set chandeliers swinging might rate as a I or II on this scale, while one that destroyed huge buildings and created panic in a crowded city might count as an X. The obvious problem with the Mercalli scale was that it relied on subjective measures of how well a building had been constructed and how used to these sorts of crises the population was. The Mercalli scale also made it difficult to rate earthquakes that happened in remote, sparsely populated areas.