"There is no place for antisemitism in our society," Johnson said on Twitter. "Ahead of Shavuot, I stand with Britain’s Jews who should not have to endure the type of shameful racism we have seen today."
Johnson did not specify what incident he was referring to.
A video posted online earlier on Sunday showed a convoy of cars bearing Palestinian flags driving through north London and broadcasting antisemitic messages from a megaphone.
Reuters has not been able to independently verify the video or when it was filmed.
Britain's Metropolitan police said: "We are aware of a video appearing to show antisemitic language being shouted from a convoy of cars in the St John's Wood area this afternoon."
"Officers are carrying out urgent enquiries to identify those responsible. This sort of behavior will not be tolerated."