Mr Galloway is running for a seat on the London Assembly
George Galloway was left "momentarily dazed" after being hit by a rubber ball while out campaigning.
The Respect Party MP was targeted while on his open-top campaign bus in Holborn, central London, during his campaign for a London Assembly seat.
The Bethnal Green and Bow MP lost his balance and hit the other side of his head on the side of the bus.
An office worker was arrested and is in custody at a police station in north-west London.
The missile - described as slightly smaller than a tennis ball and hollow - is believed to have been flung from the first floor of an office block on Procter Street, Holborn, shortly after 1400 BST.
Mr Galloway's aide, Kevin Ovenden, said: "It hit George on the left side of his head, on the temple.
"He was momentarily dazed and because of the impact of the blow he lost his balance and hit the other side of his head on the side of the bus.
"There is a nasty bruise on the side of George's head, but he will continue campaigning for the local elections."
Mr Galloway is running for the London Assembly in May's elections as part of a "progressive list".
In 2003 he was expelled from the Labour Party after urging Iraqis to resist British and US troops, and helped found Respect.
He beat Labour loyalist Oona King to take Bethnal Green and Bow in the 2005 general election, having traded heavily on his anti-war record.