Olli Lounasmaa (born 20.08.1930) is a Finnish physicist and a member of the Finnish Academy of Science and Letters (1970). He was born in Turku. He graduated from the University of Helsinki (1953), and in 1958 he received the degree of Doctor of Philosophy from the University of Oxford. In 1953—54 he worked at the University of Helsinki and in 1954—64 at the University of Turku. From 1965 he has been professor at the Helsinki University of Technology.
His main research concerns low-temperature physics. He carried out a series of works on producing and measuring temperatures below 1 K and on conducting research at these temperatures. He measured the temperature dependence of the heat capacity of helium-3 in the neighbourhood of the A transition and showed that it is a second-order transition. He obtained direct proof that below the A transition liquid helium-3 becomes superfluid. He carried out measurements of the viscosity of liquid helium-3.