Brown announced the aid at Downing Street, as he met Zimbabwe Prime Minister Morgan Tsvangirai.
The money will be channelled through aid agencies, not the government in which Tsvangirai shares power with President Robert Mugabe.
Zimbabwe’s government says it needs some $8bn (£5) to revive its shattered economy.
Brown said it would bring British transitional aid this year to a total of $60m (£36m).
Some £4m of the extra aid will be devoted to food aid and the rest to buying text books for Zimbabwean schools.
Brown was speaking at a joint news conference with Zimbabwe’s Prime minister Morgan Tsvangirai who defended his decision to share power with President Mugabe in February.
Tsvangirai said "irreversible change was now taking place in Zimbabwe towards a transition to democracy and elections".
Zimbabwean prime minister Morgan Tsvangirai is seeking financial support at a meeting with Gordon Brown in Downing Street.
Mr Tsvangirai will argue that the Zimbabwean government has made progress with reform since he entered a power-sharing deal with his former foe, President Robert Mugabe, in February.
He is also seeking to pave the way for an end to sanctions against his country, which were imposed in response to Mr Mugabe’s economic policies and political repression.
Mr Tsvangirai was heckled and jeered when he appealed to exiles in a speech in London on Saturday to return to their homeland.
Many of the crowd of more than 1,000 who gathered to hear him at Southwark Cathedral were angered when he claimed that Zimbabwe was "changing for the better".
He has taken the same message to Number 10 on the latest leg of his fund-raising tour – to help re-build his stricken country – in Europe and the US.
Mr Tsvangirai has struggled to raise anything like the money he had hoped, amid continued concern about the Zimbabwean government’s land reforms.
He has been criticised for lending legitimacy to Mr Mugabe’s rule by joining his government.
As the leader of the Movement for Democratic Change, Mr Tsvangirai has in the past been a target for Mr Mugabe’s Zanu PF militias.
But he said the country was now in "transition" and would be urging Mr Brown to support its progress.
After his talks with David Miliband at the weekend, the Foreign Secretary said the British Government was ready "to do what we can to help".
"We are encouraged by his government’s initial steps to reform the economy," he said.
"More needs to be done to restore investor and donor confidence.
"The establishment of the rule of law and transparent economic management will be essential to Zimbabwe fulfilling its full economic potential."