Donald L. D. Caspar (January 8, 1927 - November 27, 2021) was an American structural biologist (the very term he coined) known for his works on the structures of biological molecules, particularly of the tobacco mosaic virus. He was an emeritus professor of biological science at the Institute of Molecular Biophysics, Florida State University, and an emeritus professor of biology at the Rosenstiel Basic Medical Sciences Research Center, Brandeis University. He has made significant scientific contributions in virus biology, X-ray, neutron and electron diffraction, and protein plasticity.