Jan Ptaszycki was a Polish mathematician working in St. Petersburg in the period 1876–1912. In Poland, he is forgotten, and in Russia considered generally as a Russian mathematician, known there as Ivan Lvovich Ptaszycki. In Russian literature about history of mathematics,depending on the author, it is reported that Ptaszycki was a Russian mathematician or, more rarely, a Polish mathematician as, for example, in Lokot (2015, 2018). The important information about Ptaszycki comes from the obituaries of K. A. Posse (1912) and S. Dickstein (1912) and from the memoirs of I. Ja. Depman (1960), that is from people who knew him personally. We wanted to make known to the contemporary reader this forgotten Polish mathematician working in St. Petersburg. This article tries to reach his work and achievement in mathematics as well as information about his activity. After discussing his biography, we briefly discuss his mathematical work. At the end of this paper, we quote articles and books related to Ptaszycki scientific achievements and we present a full list of his published works and books, and also references where one can find information about him. .