Holder says his letter follows news reports that Snowden, who leaked information on largely secret electronic surveillance programs, has filed papers seeking temporary asylum in Russia on grounds that if he were returned to the United States, he would be tortured and would face the death penalty.
The attorney general's letter was sent to Alexander Vladimirovich Konovalov, the Russian minister of justice.
Holder's letter is part of an ongoing campaign by the U.S. government to get Snowden back.
The attorney general's letter may allay reported Russian concerns about how Snowden might be treated if he is deported to the U.S.
Some Russian politicians, including parliament speaker Sergei Naryshkin, have said Snowden should be granted asylum to protect him from the death penalty.
If Snowden were to go to a country that opposes the death penalty, providing assurances that the U.S. won't seek the death penalty may remove at least one obstacle to his return to the U.S.
"I can report that the United States is prepared to provide to the Russian government the following assurances regarding the treatment Mr. Snowden would face upon return to the United States," Holder wrote. "First, the United States would not seek the death penalty for Mr. Snowden should he return to the United States." In addition, "Mr. Snowden will not be tortured. Torture is unlawful in the United States," Holder's letter said.
The attorney general said that if Snowden returned to the U.S. he would promptly be brought before a civilian court and would receive "all the protections that United States law provides."
Holder also said that "we understand from press reports and prior conversations between our governments that Mr. Snowden believes that he is unable to travel out of Russia and must therefore take steps to legalize his status. That is not accurate; he is able to travel."
Despite the revocation of Snowden's passport on June 22, Snowden remains a U.S. citizen and is eligible for a limited validity passport good for direct return to the United States, said the attorney general.
A spokesman for President Vladimir Putin said Russia has not budged from its refusal to extradite Snowden.
Snowden, who is believed to have been staying at the Moscow airport transit zone since June 23, applied for temporary asylum in Russia last week. The United States wants him sent home to face prosecution for espionage.
Asked by a reporter whether the government's position had changed, Dmitry Peskov told Russian news agencies that "Russia has never extradited anyone and never will." There is no U.S.-Russia extradition treaty.
Peskov also said that Putin is not involved in reviewing Snowden's application or discussions of the ex-NSA contractor's future with the U.S., though the Russian Security Service, the FSB, had been in touch with the FBI.