The maverick Respect Party leader – who has threatened a legal challenge against his general election defeat – declared his bid on Twitter.
In a typically unconventional manner, the former left-wing Labour firebrand posted a picture of himself sitting on an unmade bed with his infant son and the slogan “Dad, you’ve got to do this. I’ve got your back!”.
“I’m standing for Mayor of London 2016,” he added – with an appeal for people to come forward to help his campaign.”
On 7 May, Mr Galloway was unable to hold on to the Bradford West Commons seat he snatched from his former party in a 2012 by-election, losing to Labour’s Naz Shah by a majority of more than 11,000 following a bitter campaign.
He said afterwards that he would seek to have the result set aside over claims of candidates making false statements.
A complaint against him over his use of parliamentary funds while an MP has been referred to police.
The City Hall election will take place in May 2016 when Boris Johnson – who returned to the House of Commons at the general election – steps down after two terms.
Mr Galloway revealed during the general election campaign his intention to enter the race for the top political job in the English capital should he be ousted from Parliament.
The Scottish former Labour MP famously pretended to be a cat with fellow contestant, actress Rula Lenska, on Celebrity Big Brother in 2006.