The billionaire financier George Soros is giving $100 million (£65 million) to America's leading human rights organisation in a move that will enable the group to massively expand its operations around the world.

Human Rights Watch (HRW) will increase its staff of 300 by about a third on the back of the five-year Soros grant, using the new firepower to increase its advocacy in key emerging regions in the developing world. The donation, the largest in HRW's history and bigger than any previous single grant from Soros to a US organisation, aims to help the group fulfil its ambition to boast a truly global reach.

The New York-based organisation, formed in 1978 as Helsinki Watch, has built its reputation as a leading watchdog on governmental abuses around the world. But it has tended to focus its energies on western capitals, believing the best way to improve human rights was to persuade developed countries to use their muscle to effect change.

Increasingly, though, the group has become aware of the strength of developing nations such as Brazil, South Africa and India, who may now hold more sway in their regions than Washington, London or Berlin. The Soros grant will be used to beef up operations in those countries.

"Think of Zimbabwe," Kenneth Roth, HRW's director, said. "The US and European Union all have great policies on Zimbabwe, but it's not enough: the key to changing Mugabe is South Africa."

Roth also gave the example of Sri Lanka, where both the United States and the European Union have strong human rights policies but have failed to prevent president Mahinda Rajapaksa "thumbing his nose at the West". A HRW focus on India and other Asian countries, including China, could have more impact.

Announcing the grant, Soros said he was drawn to the idea of helping the group extend its global influence as part of his philanthropic mission to use his huge wealth, obtained through interventions in the money markets, to forward the cause of open societies.

"Human rights underpin our greatest aspirations: they're at the heart of open societies," he said. "Human Rights Watch must be present in capitals around the globe, addressing local issues, allied with local rights groups and engaging with local government officials."

The grant, Soros said, would inspire a sea-change in the organisation that would result in its largely New York-based management having up to half its board members outside the US.

So far this year Soros has handed out about $800m through his Open Society Foundations. Despite the economic downturn, which has caused a dip in philanthropic giving, Soros has been increasing his largesse recently, partly, he has said, out of recognition of his age – he turned 80 last month – and a desire to distribute much of his wealth during his lifetime.

Soros was born in Hungary. He is a consummate player of international currencies, and is famous for having made about $1 billion during the Black Wednesday crisis in the UK in 1992.

His donation comes at an opportune moment for HRW. The group has had a turbulent year, sustaining criticism for its reporting on Gaza war crimes, which it said amounted to a disinformation campaign by the Israeli government. Its own co-founder, Robert Bernstein, has accused the group of bias against Israel in its handling of the Israeli-Palestinian conflict.