Esper Signius Larsen, Jr. (14 March 1879 – 8 March 1961) was an American geologist and petrologist who contributed to techniques for age estimation using the lead-uranium ratio or Larsen method and the petrological study of non-opaque minerals using optical microscopy techniques. He served as a professor of petrology at Harvard University from 1923 to 1949. The mineral Esperite (earlier called Calcium Larsenite) was named after him.
