Hiroshima Mayor Tadatoshi Akiba and Nagasaki Mayor Tomihisa Taue said at a press conference that they will establish a joint committee for a bid that will emphasize world peace.
"The Olympics symbolize the abolition of nuclear arms and world peace, and we want to work to realize our plan to host the games," Akiba said.
Taude added: "It will be a new challenge for the atomic-bombed cities."
The announcement comes a little more than a week after Tokyo lost its bid to host the 2016 Olympics. Tokyo organizers have not indicated whether they will bid again.
Tokyo Governor Shintaro Ishihara made headlines last week when he blamed his city's failure on Japanese sports officials' shortcomings in maneuvering the inner workings of the International Olympic Committee. Officials from Rio's bid described Ishihara's comments as "inappropriate.
Tokyo spent 15 billion yen ($166 million) promoting itself for the games.
A number of cities have already expressed interest in vying for the 2020 Olympics, including Istanbul, Budapest and Delhi. A host city is expected to be named in 2013, and only one city per country is allowed to submit a bid.
Both mayors are founding members of the Mayors for Peace 2020 Vision Campaign, which advocates for a global ban on nuclear arms. In a speech last month in Mexico City, Akiba said he "firmly" believed the world could abolish nuclear weapons by 2020 and suggested holding the Olympics that year in Hiroshima and Nagasaki to celebrate.
Hiroshima, the site of the world's first atom bombing on Aug. 6, 1945, is located in western Japan about 640-kilometers from Tokyo. The city was instantly flattened and an estimated 140,000 people were killed or died within months when the American B-29 bomber Enola Gay dropped the bomb in the waning days of World War II.
Three days later, the U.S. dropped a plutonium bomb on the port city of Nagasaki, killing about 80,000 people. Japan surrendered on Aug. 15, ending World War II. A total of about 260,000 victims of the attack are officially recognized by the government, including those that have died of related injuries or sickness in the decades since.
Nagasaki sits about 320 kilometers west of Hiroshima on the island of Kyushu.
In 1994 Hiroshima held the Asian Games, which brought about 7,300 athletes from 42 countries and regions to the city.
The Japanese Olympic Committee praised the two cities for their enthusiasm but noted that a successful bid would requires more than just a message of world peace.
"The concept to host the Olympics is wonderful," JOC Secretary General Noriyuki Ichihara said, according to Kyodo news agency. "But I believe it would be difficult for the IOC to accept it just on the basis of abolishing nuclear weapons."
Yeah, it's gonna be about toning that xenophobia down from 9 to about 1, Japan.