Government adviser Ramsay Jones tweeted a picture of the long-legged bird perched on top of the famous Number 10 black front door.
"Guess what just flew into Number 10", he asked, before confirming it had eventually flown away.
Prime Minister David Cameron, who is on a tour of EU capitals, was not in to receive his unexpected guest.
But it still took a look inside, Mr Jones reported:
According to the RSPB, grey herons are found around "any kind of water" - so the nearby Thames or the Downing Street garden pond might have been its next stop.
Previous wildlife appearances in Downing Street include a fox on the morning before the general election.
There have also been regular mouse sightings - including during a prime ministerial dinner.
A heron has caused a stir on Twitter after paying a flying visit to Downing Street.