Hans Waldemar Ruin (18 June 1891 – 3 November 1980) was a Finnish philosopher and writer of Swedish-Finnish extraction.

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.

Ruin was a varied writer, who authored works on topics ranging from continental philosophy to art and literature, as well as autobiographical pieces.

This was one of the reasons why he left Finland and moved to Sweden to pursue the study of aesthetics.