He said spontaneity was not in the interests of national security.
Mr Trump is known for regularly making broad pronouncements on issues of national importance on his Twitter feed.
Mr Brennan also said that Mr Trump did not fully appreciate Russia's capabilities or intentions.
"I think Mr Trump has to understand that absolving Russia of various actions that it's taken in the past number of years is a road that he, I think, needs to be very, very careful about moving down," he said.