Lem died in a Krakow hospital from heart failure "connected to his old age," the secretary, Wojciech Zemek, told The Associated Press.
Lem was one of the most popular science fiction authors of recent decades to write in a language other than English, and his works were translated into more than 40 other languages. His books have sold 27 million copies.
His best-known work, "Solaris," was adapted into films by director Andrei Tarkovsky in 1972 and by Steven Soderbergh in 2002. That version starred Clooney and Natascha McElhone.
His first important novel, "Hospital of the Transfiguration," was censored by communist authorities for eight years before its release in 1956 amid a thaw following the death of Soviet dictator Josef Stalin.
Other works include "The Invincible," "The Cyberiad," "His Master's Voice," "The Star Diaries," "The Futurological Congress" and "Tales of Prix the Pilot."
Lem was born into a Polish Jewish family on Sept. 21, 1921, in Lviv, then a Polish city but now part of Ukraine.
His father was a doctor and he initially appeared set to follow in that path, taking up medical studies in Lviv before World War II.
After surviving the Nazi occupation, in part thanks to forged documents that concealed his Jewish background, Lem continued his medical studies in Krakow. Soon afterward, however, he took up writing science fiction.
Lem is survived by his wife and a son, Zemek said. Funeral arrangements were not disclosed.