| Hans Ruin | |
| Birth Name: | Hans Waldemar Ruin |
| Birth Date: | 18 June 1891 |
| Birth Place: | Helsinki, Grand Duchy of Finland, Russian Empire (now Finland) |
| Death Place: | Stockholm, Sweden |
| Period: | 1919–1977 |
| Alma Mater: | Helsinki University Lund University |
| Spouses: | Karin "Kaisi" Sievers |
Hans Waldemar Ruin (18 June 1891 – 3 November 1980) was a Finnish philosopher and writer of Swedish-Finnish extraction.
Ruin was the son of Professor Waldemar Ruin and Flora Lindholm. He married Karin "Kaisi" Sievers (1894) in 1917, daughter of physician Klas Richard Sievers and Freifrau Karin von Bonsdorff. He had two children, Martina and Olof, and maternal grandfather to David and Marika Lagercrantz. His grandchild was also named Hans Ruin and became a philosopher.
Ruin was a varied writer, who authored works on topics ranging from continental philosophy to art and literature, as well as autobiographical pieces. Analytical philosophy, an increasingly dominating subject within philosophy at the time, felt foreign to him however. This was one of the reasons why he left Finland and moved to Sweden to pursue the study of aesthetics.
Svenska kulturfonden sponsored an award in 2003 called the Hans Ruin essay contest for essay writers. The contest is arranged every three years.