Olli Viktor Lounasmaa (20 August 1930 – 27 December 2002) was a Finnish academician, experimental physicist and neuroscientist.

He was known for his research in low temperature physics, especially for experimental proof of the superfluidity of helium-3 and also for his work in the field of magnetoencephalography.

Lounasmaa worked as a visiting scientist at the Argonne National Laboratory in the United States from 1960 to 1964 before he was invited to the position of professor of engineering physics at the Helsinki University of Technology in 1964.

He and his students played a key role in the development of the theory and technology for magnetoencephalography (MEG), opening new ways to study the brain.

With his early MEG students and postdocs (Matti Hämäläinen, Riitta Hari, Risto Ilmoniemi, and Jukka Knuutila), he published the highly cited and influential paper “Magnetoencephalography—theory, instrumentation, and applications to noninvasive studies of the working human brain” (Rev.