The Bees were beaten 6-0 yesterday at in-form Chesterfield, stretching a winless league run to nine matches.
Beadle, 48, was appointed in late August and guided the side to the FA Cup Second Round, defeated in extra-time by League One MK Dons.
In a statement, Barnet said: "The Club would like to place on record it’s gratitude and thanks to Peter for all of his hard work during what has been a tough period for the Club.
"Peter is very much someone who will always be welcome at The Hive London and we wish him well for the future.
"We will now look to make a swift appointment and the search for a new manager will begin immediately. We will provide a further update when this process has concluded".