The Reconstruction Agency study found about 90 percent of the 1,263 disaster-related deaths were of people at least 70 years old, The Mainichi Shimbun reported.
More than half of the victims, or 638 people, died from effects of the triple disaster in March 2011, the agency said. Of those, 433 died in Fukushima Prefecture, the location of the nuclear energy facility heavily damaged by the quake and tidal wave.
Of the total deaths, about half died within one month of the disaster. About 80 percent died within three months.
Fatigue from living in evacuation centers was cited as the single biggest cause of post-disaster death.
Many of the Fukushima victims died after being forced to move from hospitals or nursing homes after the reactor was damaged.
The report said 283 died because hospitals were not able to function or because the victims' illnesses worsened.
The report recommended upgrading existing laws to ensure adequate emergency food supplies, proactive action against bad weather and prevention of disaster-related deaths by providing appropriate health care.