The Internet Watch Foundation (IWF), an industry-funded body which identifies and helps remove criminal content online, said the "incredibly generous" donation would be paid over four years and allow it to hire five new staff.
Google is already one of the highest paying members of the 17-year-old foundation, contributing 20,000 pounds a year alongside other media giants BT, Vodafone, Sky, Telefonica and EE.
The donation, thought to have been agreed in the past couple of days, is an attempt to allay criticism that Google is failing to do its part in ridding the Internet of pictures of child abuse.
And culture minister Maria Miller has summoned representatives of several online giants including Google and Facebook for talks on June 17 to hear what they are doing to police their content.
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There have been two high-profile trials over grisly child sex murders in Britain in recent weeks, and the killers of five-year-old April Jones and 12-year-old Tia Sharp were both found to have searched for child porn online.
IWF chief executive Susie Hargreaves said the donation was "incredibly generous" and demonstrated "moral leadership in the field".
"The IWF are essential partners in our fight to rid the Internet of this illegal material by providing us with lists of web pages that we block from search results.
"Our donation should help them do their work more quickly and efficiently."
A spokesman for the culture ministry welcomed Google's donation but said: "It's just a first step.
"Parents are rightly worried about what their children have access to online and given the proliferation of child abuse imagery, we need a committed emphasis on tackling the problem.