The group amounts to the largest coalition yet to give its backing to an international financial transactions tax, and includes members of the UK-based Robin Hood Tax campaign, including the TUC, Friends of the Earth and ActionAid.
Their plea comes ahead of the summit of leaders of G20 economic powers in the South Korean capital, Seoul, on Thursday and Friday, when measures to stabilise the world economy and boost the recovery will be high on the agenda.
The letter, addressed to G20 leaders, including Prime Minister David Cameron and United States president Barack Obama, is signed by development, health, education and environmental charities and unions from 16 of the G20 countries.
It says a financial transaction tax would help to meet the costs "of the global financial and economic crisis, including reducing the unacceptably high rate of job loss, and achieve key development, health, education and climate change objectives in developing countries".
Brendan Barber, TUC general secretary, said: "The banks are back to business as usual with multimillion-pound salaries to the chief executives of bailed-out banks and billions in bonuses. A Robin Hood Tax would mean the world's banks paying to reduce deficits they helped cause."
Jenny Ricks, the head of campaigns at ActionAid, said: "The G20 must now turn the global economic crisis into an opportunity to help the world's poorest.
"A tiny tax on the banks could raise hundreds of billions needed for those around the world feeling the effects of a crisis they did the least to create."