Klavs Flemming Jensen (born August 5, 1952) is a chemical engineer who is currently the Warren K. Lewis Professor at the Massachusetts Institute of Technology (MIT).

Jensen was elected a member of the National Academy of Engineering in 2002 for fundamental contributions to multi-scale chemical reaction engineering with important applications in microelectronic materials processing and microreactor technology.

From 2007 to July 2015 he was the Head of the Department of Chemical Engineering at MIT.

Jensen received his chemical engineering education from the Technical University of Denmark (M.Sc., 1976) and University of Wisconsin–Madison (PhD, 1980). Jensen's PhD advisor was W. Harmon Ray. In 1980, Jensen became assistant professor of chemical engineering and materials science at the University of Minnesota, before being promoted to associate professor in 1984 and full professor in 1988. In 1989, he moved to the Massachusetts Institute of Technology.

At the Massachusetts Institute of Technology, Professor Jensen has been the Joeseph R. Mares Career Development Chair in Chemical Engineering (1989–1994), the Lammot du Pont Professor of Chemical Engineering (1996–2007), and the Warren K. Lewis Professor of Chemical Engineering (2007– present). Klavs served as Head of the MIT Department of Chemical Engineering from 2007 to 2015. In 2015, Professor Jensen became the founding Chair of the scientific journal Reaction Chemistry and Engineering by the Royal Society of Chemistry focused on bridging the gap between chemistry and chemical engineering.

Jensen's research revolves around reaction and separation techniques for on-demand multistep synthesis, methods for automated synthesis, and microsystems biological discovery and manipulation. He is considered one of the pioneers of flow chemistry.

Jensen, Armon Sharei and Robert S. Langer were the founders of SQZ Biotech. The trio, together with Andrea Adamo, developed the cell squeezing method in 2012. It enables delivery of molecules into cells by a gentle squeezing of the cell membrane. It is a high throughput vector-free microfluidic platform for intracellular delivery. It eliminates the possibility of toxicity or off-target effects as it does not rely on exogenous materials or electrical fields.

Jensen, along with Timothy F. Jamison, Allan Myerson and coworkers, designed a refrigerator-sized mini factory to make clinic-ready drug formulations. The mini factory can make thousands of doses of a drug in about two hours. The factory can allow sudden public health needs to be more easily addressed. It can also be useful in developing countries and for making medicines with a short shelf life. Chemical & Engineering News named the mini factory in their list of notable chemistry research advances from 2016.