Garland said Robert Hur, formerly the principal associate deputy attorney general under ex-President Donald Trump, would be given the title of special counsel and empowered to examine whether the cache violated any laws.
The new find — like the first from Biden's time as vice president — was uncovered in the garage of his home in Wilmington, Delaware, where he often spends weekends, the White House said.
"As I have said before, I strongly believe that the normal processes of this department can handle all investigations with integrity," Garland said. "But under the regulations, the extraordinary circumstances here require the appointment of a special counsel for this matter."
"This appointment underscores for the public the department's commitment to both independence and accountability and particularly sensitive matters, and to making decisions indisputably guided only by the facts, and the law," he added.
Garland's announcement came hours after the White House acknowledged the second batch of papers in a statement that did not address their content.
The first batch of classified documents were found at Biden's former office at the Penn Biden Center think tank last November before lawyers turned them over to the National Archives, the White House counsel's office said.
The disclosures have prompted comparisons to the special counsel investigation of Trump's hoarding of hundreds of classified materials at his South Florida beachfront home and his alleged obstruction of government efforts to get them back.
"People know I take classified documents and classified material seriously. We're cooperating fully (and) completely with the Justice Department's review," Biden told reporters.
"As part of that process, my lawyers reviewed other places where documents from my time as vice president were stored, and they finished the review last night."
The president said a "small number" of documents with classified markings had been found in storage areas and his library and that the Justice Department was notified immediately.
Biden declined to take shouted questions from the press, but House Speaker Kevin McCarthy has called for a congressional investigation into the discovery and the earlier find at his former Washington office.
Biden had told reporters in Mexico City on Tuesday he was "surprised" to learn of the original discovery on Nov. 2, saying he did not know that any had been taken there.
Hur is expected to begin work in the coming days.