Shoko Asahara's appeal against his death penalty was rejected by the top court on Friday.
"Effective today, the court dismisses the special appeal of this case," the court said in a statement read by a Supreme Court spokeswoman.
Lawyers of Asahara, 51, had argued that the former leader of Aum Shinri Kyo, or Supreme Truth Sect, was mentally incompetent and called for the case to be suspended.
Asahara, whose real name is Chizuo Matsumoto, was found responsible for a nerve gas attack on Tokyo trains during rush-hour that killed 12 people and poisoned thousands of others.
He was sentenced to death by a Tokyo court in February 2004 for murder and attempted murder.
About 5,500 people were injured, some permanently, after cult members released sarin, a lethal nerve gas first developed by the Nazis in World War Two.
Japan normally announce dates of executions, which are by hanging, in advance of them being carried out.
The nearly blind Asahara was also found guilty of other charges including a series of crimes that killed 15 people.
The son of a poor maker of "tatami" straw mats, Asahara graduated from a school for the blind before working as an acupuncturist and amassing wealth with sales of Chinese medicine in the early 1980s.
He later studied yoga and started a school to teach it, going on to set up the cult in 1987, mixing Buddhist and Hindu meditation with apocalyptic teachings.
Under Asahara, who had predicted that the United States would attack Japan and turn it into a nuclear wasteland, followers submitted to an ascetic communal life and performed rites such as swallowing water and then vomiting it up to "purify" them.
At its peak, the cult boasted at least 10,000 members in Japan and overseas, including some who had studied science at the nation's elite universities.
Raids on the cult's sprawling complexes at the foot of Mount Fuji after the subway attack uncovered stockpiles of high-tech equipment and dangerous chemicals.