Philip Siekevitz (February 25, 1918 – December 5, 2009) was an American cell biologist who spent most of his career at Rockefeller University. He was involved in early studies of protein synthesis and trafficking, established purification techniques to facilitate study of the cell nucleus, worked with Nobel Prize in Physiology or Medicine winner George Palade on cell membrane dynamics, and published extensively on the subject of postsynaptic density.