The Knesset approved Barak's appointment as defense minister on a 49-22 vote Monday, Ynetnews reported.
Barak was expected to begin work focusing on Gaza and the Hamas takeover there.
Peretz said goodbye to generals at the military event before leaving the Defense Ministry building, Ynetnews said.
After Monday night's Knesset vote, Peretz said: "I know I am being made to pay the price for past failures and I surrender to that. I ask just one thing: That you all keep your eyes open -- Knesset members, ministers and you the people of Israel."