"I do not intend to withhold briefings from any officially nominated, eligible candidate," Clapper wrote to Ryan.
Since 1952, Republican and Democratic presidential and vice presidential nominees have been offered intelligence briefings on national security after their parties' national conventions to prepare for the transition of power.
"Nominees for president and vice president receive these classified briefings by virtue of their status as candidates and do not require separate security clearances before the briefings," Comey wrote. "Briefings for the candidates will be provided in an evenhanded, nonpartisan basis."
Ryan spokeswoman AshLee Strong said he disagrees with Clapper's decision.
"We obviously disagree with the decision and want to know what precautions will be taken and what assurances the director can give that Secretary Clinton won't mishandle classified information. She has proven herself untrustworthy," she wrote in a statement.
Former national security officials have expressed reservations about Clinton's and Donald Trump's receiving classified briefings.
House Republicans leaders sent a letter on Monday to the U.S. attorney for the District of Columbia requesting an investigation into whether Clinton lied to Congress about her use of a private email server.
She told the House Select Committee on Benghazi in October that she never sent or received emails marked classified over her private email server. Republicans have expressed concern that she lied to them after Comey told the House Oversight Committee last week that some of Clinton’s emails were marked classified.
He told the panel the FBI had "no basis to conclude" that Clinton lied to the FBI when the agency interviewed her about her emails. He also suggested that she may not have understood that some of her emails were marked classified.