The article is devoted to a life and scientific activities of P.I. Walden (1863 – 1957), who was an outstanding chemist and a historian of chemistry. He took part in the foundation of Polytechnical Institute in Saint Petersburg, then was a rector of Riga Polytechnical Institute, an ordinary academician of Petersburg Academy of Science, a foreign honorary member of the Academy of Science of the USSR. The scientist is known, first of all, for his works on physical chemistry and electrochemistry of non-aqueous solutions as well on stereo-chemistry.