Giovanni Battista Grassi (27 March 1854 – 4 May 1925) was an Italian physician and zoologist, best known for his pioneering works on parasitology, especially on malariology. He was Professor of Comparative Zoology at the University of Catania from 1883, and Professor of Comparative Anatomy at Sapienza University of Rome from 1895 until his death. His first major research on the taxonomy and biology of termites earned him the Royal Society's Darwin Medal in 1896.

Grassi's scientific contributions covered embryological development of honey bees, on helminth parasites, the vine parasite phylloxera, on migrations and metamorphosis in eels, on arrow worms and termites. He was the first to demonstrate the life cycle of human dwarf tapeworm Taenia nana, and that this tapeworm does not require an intermediate host, contrary to popular belief. He was the first to demonstrate the direct life cycle of the roundworm Ascaris lumbricoides by self-experimentation. He described canine filarial worm Dipetalonema reconditum, and demonstrated the parasite life cycle in fleas, Pulex irritans. He invented the genus of threadworms Strongyloides. He named the spider Koenenia mirabilis in 1885 after his wife, Maria Koenen. He pioneered the foundation of pest control for phylloxera of grapes.

The most important contributions of Grassi are on malariology, discovering different species of malarial parasites in birds and humans, and their transmission. With Raimondo Feletti, he discovered Haemamoeba praecox and H. relictum (now under the genus Plasmodium) in birds. They correctly described Haemamoeba malariae and H. vivax (both now under Plasmodium), which became fundamental to clinical distinction of different human malaria: benign tertian caused by P. vivax, malignant tertian by P. falciparum and benign quartan by P. malariae). He was the first to describe and establish the life cycle of the human malarial parasite, Plasmodium falciparum, the most prevalent and deadliest species. He discovered that only female anopheline mosquitoes are capable of transmitting the disease.

Grassi's works in malaria remain a lasting controversy in the history of Nobel Prizes. Since the inception of Nobel Prizes in 1901 until his death, he was nominated 21 times. For the 1902 Nobel Prize in Physiology or Medicine, he was nominated alongside French physician Charles Louis Alphonse Laveran, who discovered P. falciparum, and British army surgeon Ronald Ross. He and Ross were shortlisted for the final award, but Ross who appeared to have make the least important discovery, the transmission of malarial parasite in birds, was the sole winner. Grassi, who demonstrated the complete route of transmission of human Plasmodium, and correctly identified the types of malarial parasite as well as the mosquito vector, Anopheles claviger, was denied.

Grassi was born in Rovellasca, Italy, in what is now the Province of Como. His father Luigi Grassi was a municipal official, and mother Costanza Mazzuchelli was a noted peasant of unusual intelligence. He completed elementary education at Bolchi-Stucchi private school in Saronno, and secondary education at Volta high school in Como.

From 1872 he studied medicine at the University of Pavia under professors Camillo Golgi and Giulio Bizzozero and graduated in 1878. After graduation he worked first at Messina in the Naples Zoological Station and the Oceanographic Station founded by Nicolaus Kleinenberg and Anton Dohrn where he studied Chaetognatha, then completed his training at the University of Heidelberg in Germany under the guidance of Carl Gegenbaur and Otto Bütschli. While in Heidelberg, he met a fellow student Maria Koenen whom she married in 1879.

In 1883, he became Professor of Comparative Zoology at the University of Catania, studying cestodes, the life cycle of the European eel (Catania) and the Moray eel (Rome). Also in Catania he began to study entomology and wrote a student text "The Origin and Descent of Myriapods and Insects" in addition to scientific papers. He also began to study malaria working with Raimondo Feletti on malaria, discovering the parasite species of human and bird malaria.

In 1895, he was appointed Chair of Comparative Anatomy at Sapienza University of Rome, where he would spend the rest of his life. He joined Angelo Celli, Amico Bignami, Giuseppe Bastianelli and Ettore Marchiafava, who were working on malaria in districts around Rome. Grassi was the group's entomologist. The group announced at the session of the Accademia dei Lincei on 4 December 1889 that a healthy man in a non-malarial zone had contracted tertian malaria after being bitten by an experimentally infected Anopheles claviger.