Hague called the "horrific" use of rape and sexual violence as a weapon of war "one of the greatest and most persistent injustices in the world".
From Bosnia to Rwanda and Democratic Republic of Congo, rape has been used against hundreds of thousands of women and girls - inflicting unimaginable suffering, destroying families and fuelling conflict, he said.
"To my mind, this cause is the slave trade of our generation," said Hague, who has been hosting a two-day meeting of G8 foreign ministers in London.
Flanked by Zainab Bangura, the U.N. special representative on sexual violence in conflict, and Hollywood actress Angelina Jolie, another U.N. special envoy, Hague said G8 states had agreed on six major steps to tackle the culture of impunity.
He also announced nearly $35.5 million (£23 million) in new funding from the G8 for the issue, including more than £10 million from Britain.
Declaring war-time rape a breach of the Geneva Conventions - also known as the laws of war – gives G8 nations the responsibility to seek out and prosecute perpetrators regardless of their nationality and wherever they are in the world.
The United States, Britain, France, Germany, Italy, Japan, Canada and Russia will also back an international protocol setting out ideal standards for investigating rape and sexual violence. The aim is to increase the number of successful prosecutions by collecting the strongest possible evidence.
Amnesties for sexual violence must never be included in peace treaties, the group agreed, pledging to improve training for military and police deployed to war zones. They are often the first to come into contact with survivors of rape.
‘HISTORY'S OLDEST AND LEAST CONDEMNED CRIME’
U.N. special representative Bangura welcomed the announcement. "Today we join hands and lift our voices in unison breaking silence on history's oldest and least condemned crime - sexual violence used as a weapon of war," she said.
Jolie, who recently returned from a trip with Hague to Congo, said the G8's stance was "long overdue".
"I have heard the survivors of rape from Bosnia to DRC say that they feel that the world simply does not care about them, and who could blame them?" Jolie said.
"For too long they have been the forgotten victims of war, responsible for none of the harm but bearing the worst of the pain. But today I believe their voices have been heard and that we have finally some hope to offer them."
In an op-ed ahead of Thursday's statement, Alice Allan, the head of advocacy at CARE International UK, said the success of this G8 initiative, as with all other "internationally led" initiatives, would depend on whether there was "buy-in" from countries affected by conflict.
"Badgering from the west, albeit well intentioned, can often play into the hands of countries that have a less progressive stance on women's rights," she noted.
“Some regional efforts are emerging, such as the Great Lakes declaration on sexual violence. It is support for these homegrown efforts – combined with continued backing for civil society efforts to tackle the attitudes and behaviour that perpetuate gender inequality – that will yield results in the long run."