Kiyotsugu Hirayama (平山清次) (1874–1943) was a Japanese astronomer, best known for his discovery that many asteroid orbits were more similar to one another than chance would allow, leading to the concept of asteroid families, now called Hirayama families in his honour.

A crater on the Moon is jointly named after him and Shin Hirayama. The asteroid 1999 Hirayama is named in his honour.

References

• http://www12.plala.or.jp/m-light/Nomenclature.htm (in Japanese)
• Groups of asteroids probably of common origin, Astronomical Journal, Vol. 31, No. 743, pp. 185-188 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1918AJ.....31..185H&db_key=AST&high=40daf3f6f907959), October 1918