He was found “in an area currently open to the public during the day” at about 22:20 BST on Monday, the Metropolitan Policesaid.
A second man was arrested outside the Queen’s home on suspicion of conspiracy to commit burglary.
The Met said both men were bailed and a review into the incident was under way.
A spokesman said police inquiries were continuing.
Review launched
“A review of the specific circumstances of this incident is being carried out,” he said.
“No members of the Royal Family were at Buckingham Palace at the time of the incident.”
The spokesman added that security would form part of the review.
Buckingham Palace has not issued any comment.
The palace is the best known of the Queen’s three officialresidences, serving as a family home and the administrative headquarters of the monarchy.
Security has been breached on a number of occasions at the palace, most famously in 1982 when Michael Fagan broke into the Queen’s bedroom.
The monarch woke to find Fagan, 30, sitting on her bed, and the pair reportedly chatted for half an hour before Fagan was arrested.
An escaped psychiatric patient has also managed to get in the grounds, and on another occasion a group of anti-nuclear demonstrators scaled the walls with ladders and held a sit-down protest on the lawn.
A number of tabloid journalists have also managed to gain access over the years, including Daily Mirror reporter Ryan Parry who spent two months working undercover as a footman.
