Theresa Rachel "Tess" Clay (7 February 1911 – 17 March 1995) was an English entomologist.
She was introduced to zoology by her older relative, the ornithologist and adventurer Richard Meinertzhagen, with whom she had a very close and unusual relationship.
She became the world's expert on Mallophaga, or chewing lice; however, her work is cast into question by her probable role in Meinertzhagen's many scientific frauds.
During and immediately after World War II, she worked with Victor Rothschild at MI5.