"Let's be clear - the 0.7 percent remains, and will remain", May said in a speech in her Maidenhead constituency west of London.
She said the government's commitment to spending 0.7% of national income on foreign aid "will remain".
May had been coming under pressure after refusing to reassure supporters of the aid budget during Prime Minister's Questions on Wednesday.
"As we debate the future of our country, our relationship to the European Union and our new relationship with the world, we should wear our aid budget as a badge of honour that sets a standard for others to follow", Rowan Williams, chairman of Christian Aid, said in a statement on Friday.
However she stressed that money will have to be scrutinised carefully to ensure it is being spent in the most effective way.
The prime minister said the spending commitment "remains and will remain", ending speculation that it might be dropped from the Tory election manifesto.
Earlier this week, Microsoft co-founder and billionaire philanthropist Bill Gates warned the government that reducing aid spending overseas could cost lives.
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Other countries involved in the TPP were Brunei, Canada, Chile, Japan, Malaysia, Mexico, Peru, New Zealand, Singapore and Vietnam. Mr Pence will end his day of official engagements by meeting Opposition Leader Bill Shorten and business leaders in the city.
'I'm very proud of the record we have, of the children around the world who are being educated as a result of what the British taxpayer is doing in terms of global aid.
She added: "The ability we had to be able to help in the Ebola crisis, the work that we've been doing supporting Syrian refugees - I was in Jordan a couple of weeks ago, in a school, meeting some youngsters who are being given a good-quality education".
Major British charities have urged political parties not to scrap a pledge to spend 0.7 percent of the country's national income on aid as they gear up for a snap election set for June 8.
"UKIP would cut the Foreign Aid budget by at least £10 billion per year leaving enough to cover emergency aid, child inoculations and clean water projects".
The PM appears to have heeded the advice of Lord Williams, who is chair of Christian Aid.
Over $18bn from the United Kingdom taxpayer was used to support the world's poorest people previous year.
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