Mohiussunnath Chowdhury, 27, was unanimously acquitted by a jury at the Old Bailey in London of one charge of preparing acts of terrorism, following a retrial. He saluted the jury before leaving the dock.
A Crown Prosecution Service spokesman said Chowdhury had been charged with "the most appropriate offence" and would face no further prosecution over the incident.
Two unarmed officers suffered cuts to their hands when they disarmed him outside Buckingham Palace in August 2017.
Chowdhury, who had driven from his home in Luton, repeatedly shouted "Allahu Akbar" (God is great) as he grappled with the cops.
The jury also heard he supported terrorist acts perpetrated by Daesh and had sent a suicide note to his sister "expressing hate of the Queen and her soldiers."
Jurors were also shown a number of drawings he made, including one in which a jihadist gunman kills a police officer outside 10 Downing Street.
But during the trial Chowdhury said he wanted to be killed by armed police and had no intention to hurt anyone.