The Justice Ministry said the executions were carried out early Friday in Tokyo. Former yakuza members Katsuji Hamasaki, 64, and Yoshihide Miyagi, 56, were convicted of shooting to death two rival gangsters outside a restaurant in Chiba, just outside Tokyo, in 2005. The victims were shot several times each.
Justice Minister Sadakazu Tanigaki said it was "heinous and brutal" and the two deserved to be executed. He said he took into consideration the feelings of the victims' families and court documents when ordering they be put to death.
Human rights group Amnesty International Japan condemned the hangings. Japan and the United States are the only Group of Eight countries that have not banned the death penalty.
Capital punishment in Japan has been widely criticized by human rights groups, but polls show the public supports it. Japan's government releases few details about executions and inmates are not informed of their pending execution until immediately before it is carried out.
Executions in Japan are done by hanging. Japan executed three inmates in February. It now has 134 inmates on death row.