Thomas Harrison Montgomery, Jr. (March 5, 1873 – March 19, 1912) was an American zoologist who made important contributions to cell biology–especially in chromosomes and their roles in sex determination–as well as the biology of birds and several groups invertebrates, naming many species of ribbon worms, rotifers, and spiders. He studied in Berlin before becoming a researcher and professor at the University of Pennsylvania, where he primarily worked until his death at the age of 39.