John David Linsley (12 March 1925 – 15 September 2002) was an American physicist who performed pioneering research on cosmic rays, particularly ultra-high-energy cosmic rays. He did his most significant work from 1959 to 1978 using a ground-based array of detectors at Volcano Ranch in New Mexico. He is best known for being the first to detect an air shower created by a primary particle with an energy of 1020 eV. This was the highest energy cosmic ray observed up to that point. Linsley's observations suggested that not all cosmic rays are confined within the galaxy and showed the first evidence of a flattening of the cosmic ray spectrum at energies above 1018 eV.