Mr Webb was selected by party members and members of the public who took part in an online vote open to anyone who lives, works, owns property or visits London.
His candidacy will be officially launched at the UKIP annual conference in Eastbourne later this week.
Webb said he was “delighted to have won the nomination” and said he would now “get on with the task of setting out UKIP’s vision for London.”
“I believe that Londoners are crying out for change especially in light of the recent riots and the less that helpful comments of both Boris Johnson and Ken Livingstone.”
UKIP leader Nigel Farage said Mr Webb would be “an excellent mayoral candidate”.
The party has enjoyed previous electoral success at City Hall.
In 2004 it won two seats on the London Assembly although its members, Damian Hockney and Peter Hulme-Cross, subsequently left and formed the One London party.
The other declared candidates in next year’s Mayoral elections are: Boris Johnson (Conservative), Jenny Jones (Green Party), Ken Livingstone (Labour) and Brian Paddick (Liberal Democrat).
