After running a pro-Palestinian campaign, Galloway won over many of Rochdale's Muslim community by attacking both Labour and Britain's governing Conservatives for supporting Israel in its war against Hamas, making a foreign conflict the major issue - unusual in a by-election where local concerns usually dominate.
Elected to parliament for the seventh time, Galloway will be an irritant to Labour, a party he once belonged to before being ejected for criticising then-prime minister Tony Blair over the Iraq war.
His victory underlines the divisions in Britain over the Israel-Hamas war, which is in its fifth month and has brought protesters onto British streets in support of both sides.
More than 30,000 people have been killed during Israel's military campaign in Gaza, Palestinian health authorities say.
This follows an attack by Hamas militants in southern Israel on October 7, when Israel says 1200 people were killed and 253 taken hostage.
British MPs have faced threats to do more to bring an end to the fighting in Gaza and with the national election later in 2024, Galloway's return to parliament will be short-lived but explosive.
"Keir Starmer and Rishi Sunak are two cheeks of the same backside and they both got well and truly spanked tonight here in Rochdale," Galloway said in his victory speech on Friday.
Galloway won 12,335 votes compared with 6,638 for second-placed David Tully, an independent candidate.
The former Labour candidate, Azhar Ali, came fourth after the opposition party pulled its support from him over a recording espousing conspiracy theories about Israel.
The leader of right-wing Reform UK, Richard Tice, said the campaign had been marred by intimidation and abuse of his party's candidate and campaigners. His party's candidate came sixth.
Turnout was low at 40 per cent.
It will be the first time Galloway's left-wing Workers Party of Britain has been represented in parliament.
For some in Rochdale, a former cotton mill town near to Manchester, the so-called by-election, triggered by the death of Labour MP Tony Lloyd in January, had failed to offer them a clear choice of someone determined to help their town, ranked in the top five per cent most deprived English local authorities in 2019.
Galloway also campaigned to reinstate maternity services in Rochdale but it was his message on Gaza that rang loudest.
It is a headache Starmer could do without. He has struggled to maintain party unity over its position on Gaza.
