Ioannis "John" N. Hazzidakis (Ιωάννης Χατζιδάκις, or Hatzidakis or Chatzidakis, April 13, 1844 – 1921) was a Greek mathematician, physicist, author, and professor.

He also studied with famous German mathematicians Ernst Kummer, Leopold Kronecker, Karl Weierstrass.

He completed his basic education in Syros and from 1863 he studied mathematics at the National and Kapodistrian University of Athens.

He graduated with a Ph.D. in Mathematics in 1868 and went on a scholarship from the University in Paris and Berlin to continue his studies.

[7] He was a student of the Paris school of differential geometry and of Karl Weierstrass in Berlin.