The sarin strike killed 12 people and injured thousands more.
It shocked Japan and shed light on the fanatical Aum Shinrikyo group, which was obsessed with chemical and biological weapons.
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Eleven other Aum members have received death sentences, though none have been executed pending appeals.
Asahara, whose real name is Chizuo Matsumoto, stood passively and said nothing as he was found guilty of all 13 charges of murder and attempted murder.
Thousands of people arrived to try and get a courtside seat; spectators were reportedly chosen by lottery.
The judges in the Tokyo District Court rejected defence arguments that Asahara had lost control of his followers by the time of the 1995 attack.
His crimes included ordering another sarin gas attack in Matsumoto, Nagano Prefecture, in 1994, which killed seven people, and the killing of several wayward cult members or their relatives.
But it was the Tokyo subway attack at the height of the city’s rush hour, which most shocked Japan. Survivors still suffer from headaches, breathing troubles and dizziness.
“I can’t think of any other sentence but death for Asahara,” said Yasutomo Kusakai, a 22-year-old college student outside the court.
“Many people were killed, and he’s supposed to be the mastermind of the crimes that affected the society in a big way.”
The verdict is the culmination of a nearly eight-year trial, during which Asahara has remained largely silent.
It is still not clear exactly why Asahara ordered the Tokyo attack. The group mixed Buddhist, Hindu and Christian tenets and believed some kind of Armageddon was imminent.
The group had also begun to feel threatened by the police at the time of the subway strike and some analysts believe it was in part designed to delay and confuse the authorities.
Aum is still operating, albeit under the new name of Aleph and with a supposedly benign new remit. However, the Japanese police still monitor it closely and believe it is still dangerous.