Drew Weissman (born September 7, 1959) is an American physician and immunologist known for his contributions to RNA biology. Weissman is the inaugural Roberts Family Professor in Vaccine Research, director of the Penn Institute for RNA Innovation, and professor of medicine at the Perelman School of Medicine at the University of Pennsylvania (Penn).

Weissman's work underlies the development of mRNA vaccines, the best known of which are those for COVID-19 produced by BioNTech/Pfizer and Moderna. With biochemist Katalin Karikó, Weissman received the Nobel Prize in Physiology or Medicine in 2023 "for their discoveries concerning nucleoside base modifications that enabled the development of effective mRNA vaccines against COVID-19". Weissman has been a recipient and co-recipient of numerous awards, also including the prestigious Lasker–DeBakey Clinical Medical Research Award. In 2022, he was elected to the National Academy of Medicine and the American Academy of Arts and Sciences.

Weissman was born in Lexington, Massachusetts, on September 7, 1959, to Hal and Adele Weissman. Hal is Jewish and Adele is Italian. While his mother did not convert to Judaism, he grew up celebrating all the Jewish holidays. He grew up in Lexington and attended Lexington High School, graduating in 1977.

Weissman received his B.A. and M.A. degrees from Brandeis University in 1981, where he majored in biochemistry and enzymology and he worked in the lab of Gerald Fasman. He performed his graduate work in immunology and microbiology to receive his M.D. and Ph.D. in 1987 at Boston University. Afterward, Weissman did a residency at Beth Israel Deaconess Medical Center, followed by a fellowship at the National Institutes of Health (NIH), under the supervision of Anthony Fauci, then director of the National Institute of Allergy and Infectious Diseases.

In 1997, Weissman moved to the University of Pennsylvania to start his laboratory in order to study RNA and innate immune system biology. He is now the Roberts Family Professor in Vaccine Research at the university.

At the university, Weissman, an immunologist studying vaccines, met his future colleague and collaborator Katalin Karikó at a photocopier, where they sympathized about the lack of funding for RNA research. At the time, Karikó had been trying RNA therapy on cerebral diseases and strokes. Immunologist Weissman began collaborating with biochemist Karikó, who switched her focus to the application of RNA technology to vaccines. Weissman's support was critical in helping Karikó to continue and extend her research. Slowly they began to move the technology forward, solving problems one at a time. On the difficulty of gaining funding and recognition for their work, Weissman has commented "We had to fight the entire way."

One of the major scientific obstacles they faced was that the RNA caused unwanted immune and inflammatory reactions as adverse side effects. Beginning in 2005, they published several landmark studies that used synthetic nucleosides to modify the RNA to prevent its degradation by the body. This breakthrough laid the groundwork for the use of RNA therapeutics, though the study received little attention at the time.

Weissman and Karikó overcame another major obstacle by developing a delivery technique to package the mRNA in lipid nanoparticles, a novel pharmaceutical drug delivery system for mRNA that protects the fragile molecule until it can reach the desired area of the body. They demonstrated the effectiveness of the delivery system in animals.