It said CIA officers acted reasonably in searching computers after concerns they contained classified material.
The finding contradicts allegations from lawmakers and an admission from the agency's own inspector general.
The allegations came during the intelligence committee's investigation into claims of torture by the CIA.
An internal board launched an inquiry after the CIA appealed against findings by David Buckley, the agency's inspector general, that employees had "improperly" gained access to computers used by Senate staff.
It concluded that although there had been some "inappropriate access" to a secured network being used by Senate workers, there had been no wrongdoing and no law violated.
Dianne Feinstein, the chair of the Senate intelligence committee, who has clashed with the CIA over the issue, rejected the board's findings.
"I'm disappointed that no-one at the CIA will be held accountable," she said in a statement.
"The decision was made to search committee computers, and someone should be found responsible for those actions," she added.
The board's conclusion comes a week after Mr Buckley said he was stepping down from his post, but officials said his resignation was unrelated to anything he had investigated.
The Senate staff had been using computers at a CIA facility to examine documents related to the agency's interrogation practices following the 9/11 attacks on the US.
Those documents formed the basis of a damning report, released in December 2014, which found the CIA had carried out "brutal" interrogations of al-Qaeda suspects.
A CIA internal watchdog has cleared agency officers of improperly accessing the computers of members of the US Senate intelligence committee.