Hans Jacob Nielsen (2 September 1899 – 6 February 1967) was a Danish featherweight and lightweight boxer who competed in the 1920s.
He was reigning world champion between 1924 and 1928 after winning the gold medal in boxing at the 1924 Summer Olympics in the lightweight category, defeating Alfredo Copello in the final.
His first Olympic appearance was in 1920 when he was eliminated in the second round of the featherweight class after losing his fight to James Cater.
In 1928 he tried to defend his Olympic title in the lightweight division but lost in the semifinal to the eventual gold medalist Carlo Orlandi. He also lost the bronze medal bout to Gunnar Berggren and finished fourth.